I made a giveaway, but I'm unsure how to send the game. Help?

Yepperz

Send it to the email address or add the user on Steam to tell the winner the key by chat.

you might want to send him an email just saying he's won (without the key) first. Some people don't check their email and no point sending them a key if they're not going to read it.

Steam chat is indeed the best way to send it.

add him to steam and talk with him, its more friendly