something i bought from green man gaming and wish to activate on steam

1) If it's Steam-redeemable, it will say that it requires Steam DRM. It's hard to miss.

2) The key will be on your key page in your game collection. It's easy to find.

3) Steam games bought from other sites work just the same as they would otherwise.

Err if you go on your account you can usually see the key, but they should tell you if it's steam redeemable beforehand. What game is it?

If it activates on Steam, it will say so on the GMG store page. For example, from here. After purchase, you will receive a key in the confirmation e-mail, and it will always be available from here. A game bought from GMG, activated on Steam, will still have all the functions (achievements, Cloud services) as if you had bought it from the Steam Store. Only difference is if it's a pre-order game, you won't get any of the pre-order bonuses listed on the Steam page.

It tells you on the page if it's activatable or not.
You gotta be careful of GMG deals if you want Steam games.