how to send game to winner?

You can add the winner to your steam friend list and trade him the game or key, or send an email to him with the key.

When the giveaway is over, I think you're meant to either add them on Steam and gift the game to them (if it's a Steam gift) or email the key to them (if it's a retail/bundle key).

Add the winner on steam and send him the key and ask him to mark the gift as received once he is done activating the key...

Click here. Click on the Gift Sent message to see the winner's profile and e-mail. Then either e-mail the key or add him in steam and send the key via steam chat.

thanks guys