Trade noob: how do I start, what do I need to know?

1) if you want to trade keys - SteamTrades. There are other great sites - for example TF2Outpost or Dispenser.TF but these are mainly for gift trading.

2) TF2 keys makes things easier - 1 common currency for all trades. Majority of your potential traders won't have the game you requested, but all of them will have TF2 keys. If you request specific ames for trade instead of general crrency you cut out 95% of your potential traders because they siply don't have the game you requested in their stock.

3) Trading keys is not supported by Steam. You will have to "go first". AKA - you send the ey first, trader activates it and if it works pays you. To avoid scammers trade this way only with high rep traders. And be awae that key trading is done on your own risk.

4) As you have not traded before you will have 0 rep and will not be considered very trustable - so there's 99% chance that you will be required to go first. If you trade be sure to check steam user you're going to trade with - especially if he's not impersonating some high-rep trader being a scammer himself. Go to his ST profile, click to go to Steam Profile and mae sure it's the same profile ID that the one that added you for trade.

Steamtrades is a good place to start trading. Only deal with people with good amount of rep. Also check if the user who added you is infact the user with the rep and not an impersonator.
You can specify what games you have and want but it can be difficult to find user who has the game you want and wants the game you have. So you can go the key route. You can ask for TF2/CSGO keys for your game and then trade them away for a game you want or sell them for Steam wallet.
You'll have to use Steam chat for trades. People won't be willing to go first because of no rep so you'll have to find someone trustworthy. See Steamtrades rep, steam level etc.

In my 10 and only trades done in Steamtrades, it involved GMG keys from me and steamgifts from traders. And I only have 10rep.
People with keys always(usually) go first when they have lower rep than the person trading with in most cases.
Lot of trust, cross-referencing SteamRep(though it's not very useful from what I hear), profile checking plus common sense.

*Also interaction is key, try to make small talk but I hear some traders hate it and want the deal done quickly. Completely situational.

Stemtrades, maybe /r/SteamGameSwap are the places to go when trading. outpost.tf no so much, since you're not trading inventory items.

TF2 (and other) keys are used since they have a somewhat fixed value ($2-2.5). It's probably easier to trade your game keys for tf2 keys and then trade those for that game you want than directly finding someone who would trade you a specific game for other game keys. Unless it's all from bundles, in which case those trades are not as hard to find.

Since you have no rep (ST and SGS are valid sources of rep, Steam profile is not), you'll probably have to go first. Meaning -> you post your key in chat, the other person activates it, and then they send you their part of the deal via chat/trade etc. Try trading with high rep (50+) users before you build up some rep yourself. This minimizes the chance of getting scammed. Once you have accumulated some rep, you can ask for the other party to go first in a trade.

Always check if the person you're trading with is who the claim they are (Enhanced Steam addon will help you find their ST profile, see if it leads to the same account), impersonators are sadly not a rare thing. Also, check if they are marked as a scammer on SteamRep (another thing more easily found with Enhanced Steam installed).

Take screenshots. A lot of screenshots. Of the trade thread offers, chat leading up to trade and trade itself. Hopefully, you won't need them as proof ever (of scam, scam attempt, other party breaking an agreement etc), but you never know.

This is not so hard to understand... This isn;t rocket sience.
Only thing I can say be wary of scammers(links, you going first with game key, people offering you like a lot in paypal(they're imposters, sometimes))
Just a tip. When someone gives you really good offer,but you have to go first in trade and he shows that his steamtrades profile has high rep, just check, if he is the one added in your friend list..(They have changed their picture and nickname to the high rep persons)
You can check, if they are legit 1.by clicking on steamtrades profile view steam profile, there you check, if you can send him a message or if it promotes to add friend. He will obviously say that it;s a bug(maybe it is, but better not trust it).2. If you have enchanted steam addon, on his steam profile there you can click on steamtrades profile and check, if the profile he gave you is the real one.
BTW, check steamrep too.

So if you're suggesting TF2 keys, what values should I ask for or can expect? For example for Thief - The Master Thief Edition or Nosgoth Founders - Warlord Pack?

i rarely use steamtrades because i prefer tradable gifts that are easier but already bought some steam keys as well and some times people with 50 positive rep and no negative offer to go first with the key and i always get stuck when they say that because usually lowest rep goes first but obviously that when i had 5 rep or less i would not even ask to the other person to go first otherwise he/she will think that i'm a scammer and probably get blocked
if you want to sell on steamtrades with no rep you should first try to trade something easy like trading gifts and at at the end ask to leave you rep and get at least 10 , of course that you can always try to sell with no rep but some scammers are smart and use alt-accounts and/or friends to +rep them selfs, they will always try to convince you to go first because you have less rep then them, ONLY GO FIRST WITH PEOPLE THAT HAVE AT LEAST 20~30 POSITIVE REP AND NO NEGATIVE and obviously check their rep about steam keys trades, install Enhanced Steam, it will allow you to check the real steamtrades profile of who you are trying to trade to avoid impersonators, always ask to leave a coment on your trade insted of directly adding you to easily check
REMEMBER that this is my opinion as a seller, as a buyer you should always check whos selling what you want/need for the cheapest price and who as the higher rep, if you find someone with 200 positive rep or more you always offer to go first because those are always very legit people and sometimes they have negative rep but if you check them usually are from haters/mad people, just click the profile of the mad person and check if someone leaved details about him (sometimes they are legit people that dont understand how things works)

about prices you can actually just say that you are looking for offers, most people will offer you steam keys or humble bundle and indiegala gifts from bundles for profit but there are always people that also have tradable gifts or other decent steam keys (obviously never accept dayz, rust and csgo steam keys ಠ_ಠ of course that sometimes they are legit, check rep first), you can also check for other people's prices on your games and have the same or lower but lets be honest here, it's a steam key from a person with no rep so only so you will get many scammers, your best bet is to try to get to trade with someone with allot of rep, it will not be easy but its not impossible and offer to go first (very important) to be sure the you aren't gonna get a negative rep for trying to scam also to make the other person to trust you
i think that's it, merry christmas :D

BTW, if you have really high rep everyone will buy from you.. Well I like put a price 3 keys, while the higher rep one has 4 keys. Everyone goes for the higher rep one, even when the price is higher... Sad truth..

You also should use multiple trade sites to maximize your profits and minimize loses like i. For example using TF2WH for buying/selling and other sites for selling/buying. Backpack tf,TF2Outpost and Bazaar tf is good. CSGOLounge and DOTA2Lounge are also if you want cross game trading specialist like i. And even person with +reps are still consindered suspicious,because i seen how players earn +reps on steam profiles,and in turn those guys was actually scammers and such. I even had a trader like this.

Trading with game keys, Steamtrades is pretty much the best place to go to.

While most of the time it will be easier to trade your game(s) for TF2 keys and turn those keys into games you'd be interested in, you might happen to find someone looking for you game and who happen to have the game you want. Look for that first and if you can't find anyone, then go with the second option. (For example, on my ST thread I'm looking for Thief and have Tomb Raider, so that would be a match to at least one of your games.)

Stick to people with high reputation especially in the beginning. Until you get the hang of things, you're at least less likely to run into a scammer that way. Always double check to make sure whoever you're speaking to is the same person that owns the rep page. Always follow the link from the ST profile to the user's account.

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I'm still kind of a noob (a touch under 50 rep), and one thing I've done when trading keys was to look for people who had their own threads, saying they wanted my game(s) rather than makimg a thread to advertise.

Yep, it can take longer to find someone, but I think it's safer. You get the benefit of being able to fully screen your buyer and can really take your time over it before you even make contact with them.

On top of that I also (righly or wrongly) believe that scammers would for the most part seek to scam anyone they could. If a person has named the games they are interested in and were to actually be a scammer, they'd be reducing their potential number of victims. As such, I think it's less likely that they'd be a scammer than a random person who replied to a thread of mine, but maybe that's what they want me to think, lol. I'm not saying that people with their own trade thrteads are instantly trustwothy, and I still very much do my homework.

imho your best bet would be go to st, look for threads offerting tomb raider key from people in the range of 50 rep and offer your thief key. if you come to a deal (check the other profile against impersonation/scamming), you'll have to go first with your key but if the other party is trustworthy you won't have problems. i guess nobody will give you tf2 keys for a thief key since there are many around and for cheap (i might be wrong). nosgoth, last remnant or guardian of light keys look to me harder to trade or anyway cheaper (again i might be wrong). i wouldn't bother creating my own thread for now, better result will come looking for the correct thread from the correct trader. gl:)

Thanks for all the help guys. Looks to me like the easiest course of action would be not to try to trade. Might as well just keep Thief and make giveaways from the lower cost games.