Welcome to the forum, Android and smartphones in general.
As already stated, the update should already be loaded or prompt you to install shortly after you boot up the phone the first time.
Here are a few threads to look at to get you started on the path of Android enlightenment:
http://androidforums.com/htc-droid-eris/53708-everything-you-need-know-about-your-htc-eris.html
http://androidforums.com/htc-droid-eris/68845-must-have-apps.html
Also, not to contradict surgeon0214 in any way (I also think ADW is pretty nice), but I would suggest running your phone stock for a little bit before you start changing things. This is good for finding bugs as well as just getting a good overall feel for the phone. Once you start to find things that you don't like or wish to improve upon, then you can start tweaking the phone as much as you want. That doesn't mean avoid installing apps of any kind, I would just avoid the things that will replace stock items for the time being.