First time creating a giveaway, how do I sent a Steam key to someone that has won the giveaway?

You like send him the key and stuff

Click on the blue ribbon where it says that your giveaway has ended and click on send gift button and you will see winners email. Compose a neat little email with steam key included or simply add the winner on steam and send him key through chat.

Either:
a) It should show they're e-mail address in the "Your giveaways" section. E-mail them the code. (Remember to screenshot the sent e-mail in case they claim they never received it.)
b) Add they're profile on Steam (watch out for impersonators) and give them the key through chat, again ensuring you screenshot that you gave them the key.

After it's all over, there's two ways:

1. Go to the link you'll see in green at the top of each page and get the user's email address, and then send them the code through email, and make sure you select "Gift Sent" once that's done.
2. Go to the link you'll see in green at the top of each page and then find the user's Steam account (go to their SteamGifts account then click their Steam account), and send them a friend request. Once they add you, congratulate them and send them the key, then click "Gift Sent" on SteamGifts.