How do I know when updates are available for my phone? I've got an Optimus T, and being new to the world of Android, I'm confused as to how I'm supposed to update my operating system. 

The phone is carrier locked to T Mobile. Does this mean that I can only get updates from them? Can I tell the phone to look on another server? How can I tell if I have the latest build of Android that's available for my device?
To check for phone updates, go into Applications > Settings > About Phone and tap System updates or check for updates. However, if a new update comes out, then you should get a notification informing you so and giving you update instructions. If you root your phone, then you could install unofficial updates that are not from T-Mobile. There are instructions at http://forums.androidcentral.com/t-mobile-optimus-t/43138-how-root-lg-optimus-t.html if you want to do so.