https://www.humblebundle.com/store

Just go there, queue it up for Steam and just grab it (the key) by the 21st into your Steam account and you're set.

They also have some insane deals going right now depending on your tastes from 40% off the Kerbal Space Program to like 80% off the entire Sonic Hits Collection for $28 instead of $113 for 17 games.