My top three are, in almost no particular order;
Buy a wort chiller. This is crucial for lowering your wort to pitching temps very quickly and easily. I built my own in no time and I wish, more than anything else that someone would have told me this when I started.
Sanitize everything. Don't cheat. Do it, and that does include your hands.
RDWHAHB. Seriously. I can't tell you the number of times a beer that was a bit...off came together and was great after I let time do it's magic. There is a reason everyone here says RDWHAHB, it's true. When one thinks of beer, normally the images associated are thos of relaxation, friendship, sports etc. Keep those images in your head, and relax don't worry have a home brew. It's good advice, and true!
Welcome to the addiction!
-E