How do i give a gift to a winner ?

Read FAQ.

Here you go, this should answer all of your questions. ^^

When giveaway ends, you get an email address for the winner. (On created page) and then you go to your Steam Client then your Inventory. Click the gift you should send, then click send gift. Choose send by email, enter email address of winner. Enter 'stuff' into the boxes. Send the gift. Feel good.

If it is a key you are giving, create a normal email and put the key in there.

Go to your inventory and find your Steam inventory, when you go on there you'll see Portal 2 and click on it, then you'll see send gift and then add in the email of the winner and send. Then wait for response until he/she marks it as received.