How to know a key form my game of steam?

if it is already in your steam libary, you can do nothing

You just can't.

You can not give away games that are already in your library.

If you are talking about a gift in your inventory you also can't. You can send the gift to your friend's email which will let them claim it and add it to their library though.

Ok, so if I want to make a Giveaway for example, I should buy a new game, not one of mines?

Damn. Ain't seen someone not know this in a long time.