Contribute

You don't need keys to contribute, you can also use Steam gifts. If you do wanna give away keys, Fanatical and Humble Bundle are your best sources. You cannot convert keys from Steam directly.

FAQ

How do the "Gift" and "Key" options work when creating a giveaway, and sending my gift?

When creating a giveaway you have the option to choose either "Gift" or "Key". The two are interchangeable, and after your giveaway ends, you can add or remove keys regardless of the option you select when creating your giveaway.

    Gift
    You'll want to choose this option when you plan to send a Steam gift to your winner, or if you would like to add keys to your giveaway after it ends. Once your giveaway is complete, you'll be able to see the winner, along with their email address. You can then choose to either send the winner an email, or a message on Steam to deliver your gift. Once the gift is sent you'll want to check the "Sent Gift" checkbox next to the winner as a reminder that you're finished with the giveaway. If you prefer to send a Steam key through our site, you can click the "Add Key" button that's displayed beside the winner. You can then enter a key and click "Save" to assign the key to the winner. At that point, the key will not yet be visible to the winner. They will only be able to see the key on our site when you check the "Send Key" checkbox. If you uncheck that box at any time, they will no longer be able to see the key. An icon with an eye will appear next to the key once it has been viewed by the winner.

    Key
    If you choose this option, you will be required to enter one or more Steam keys when creating your giveaway. Once your giveaway is complete, you'll be able to see your winner, along with their email address. You'll notice the key you previously entered is displayed next to their username, along with buttons to modify or delete the key if necessary. At that point, the key will not yet be visible to the winner. They will only be able to see the key on our site when you check the "Send Key" checkbox. If you uncheck that box at any time, they will no longer be able to see the key. An icon with an eye will appear next to the key once it has been viewed by the winner.
    If you create a giveaway for 50 keys or greater, the "Send Key" checkbox for each winner will be automatically selected. This means the keys will be instantly available to your winners the moment your giveaway ends. This is done to make the experience more convenient when managing giveaways with a higher number of keys.

You can purchase and send a gift from Fanatical, HumbleSTORE, or by becoming a Friend on Steam.
(Commonly used methods)

how to convert a steam game to a giveaway key

Do you want to convert a game from your library to a key to give away?
If you are thinking along these lines, it is not possible, you can not giveaway games registered to your Steam account.
If you are not thinking along these lines, sorry for the spam.

As you do not have any gifts in your Steam inventory, I assume you want to give one of the games from your Steam library. As Oppenh4imer mentioned above, this is not an option.
Most of us buy game bundles from Humble or Fanatical or other similar sites. The games are delivered as keys which can be activated on your account. We are often left with extra keys for games we already own or games we are not interested in owning, and those are the keys used for the majority of giveaways on this site. You can also give gifts through Steam but this is more complicated.
Also noteworthy is the reduced value of many games. If a game has been sufficiently discounted at any time, future giveaways will give the gifter reduced credit for their giveaway, and games that have been free will not reward the gifter with any credit at all. When you create a giveaway, the game's title will be marked with 1 asterisk if you get reduced credit for the giveaway and 2 asterisks is you get no credit for the giveaway. If there are no asterisks, that means you get full credit for the giveaway. You can see all these options in the following screenshot: https://imgur.com/3xHPIAb

i didn't mean converting a game from my library, i wanted to see if i can buy a new game and gift it.
thanks everyone for helping me, i will try and get them from Humble or Fanatical. 😀