Please read the FAQ

http://www.steamgifts.com/about/faq

System choose random winner and you must give him this game.

Is pretty simple, you create the giveaway and the system will choose ramdonly a winner through all the participant of the GA, when that happens you will be notified in the giveaway button, then you will give the game to the winner through email (the email will be showed) or through steam if you the game is a gift D:

And thanks for the GA D:

I hope you realize that by creating this giveaway it means that YOU are giving this game to a random winner that will be selected. You also have to supply the game yourself, so you must have an extra unused copy. If that's not what you meant to do then you should delete this before it closes. I would also suggest reviewing the FAQ and guidelines. They should answer most of your questions.

Maybe read the rules before creating one?

You'll win points, based on the game's point value, for which you'll get higher contributor levels on Steamgifts. Which will mean you'll have access to more and more giveaways.

FYI:
http://www.steamgifts.com/discussion/RqwrY/contributor-levels