I think most of us have learned Android by trying everything until something breaks.
On just about every screen in every app, I try the menu button and try long-pressing things to see what type of context menus I get. If you don't do anything to root your phone (like using Visionary) then you really can't break anything.
For extreme customizations you would have to wait anyway since there aren't any custom ROMs our for mT4G yet. But, you can go as far as putting a different launcher onto the phone like Launcher Pro or ADW (but I really like the HTC Sense customizations so I'm sticking with stock on that one).
I think you've already found the best Android resource now that you are reading XDA. Don't be afraid to ask questions if you want - just try a search first so you don't get flamed. LOL
I also read the T-Mobile mT4G forum regularly but that generally covers the less sophisticated types of questions if you know what I mean.
Good luck and have fun!