If i create a giveaway, how will i give the winner the game?

http://www.steamgifts.com/forum/X8ct9/the-new-gifters-guide-to-gifting

Steamgifts will give you the winner's email address after the giveaway, email it to them

Or you can add the winner on steam (you can check his steam profile) and, if it's a steam key or bundle gift link, put it as chat message, or if it's a Steam Gift, trade it over to him

The link above goes more in-depth