I'm planning on building my PC sometime this weekend, I would have built it last weekend but Amazon supersaver shipping is dreadfully slow.

Anyways, I bought a physical copy of Skyrim from Amazon (should ship in along with my hard drive and OS) instead of Steam because it was slightly cheaper. So after I register my game with Steam how do I update it, how do I select the "automatically update" feature to be on, and how do I make sure my game is updated to the latest patch?

Steam games update automatically by default, you dont have to do anything.

IIRC, the option to always keep the game up to date is the Steam default; once you've activated your copy of Skyrim through Steam, it should automatically search for updates (you'll get a message if it's doing so).

If it doesn't automatically update, open your Steam library, right-click on Skyrim and select Properties from the menu. Click the Updates tab, and then choose the option to "Always keep this game up to date." You may have to restart Steam for the change to take effect.

Start steam, go to your game library, right click on The Elder Scrolls V:Skyrim, and select properties. In the window that pops up, there's a tab labeled Updates. Click on that. The first section there is automatic updates. Make sure it says "Always keep this game up to date"; change it if it does not. Like the previous posters, I think that is the default behavior anyway. Every time you start Steam, it'll check all your installed games that are set to automatic updates, and will download files as needed. It may take a while, depending upon how much needs to be updated and how fast your connection speed; several of the Far Cry 3 updates have been 200 meg + patches. DLC is also automatically downloaded, and those can be much larger.