I friggin LOVE mountain biking. I have an '06 Iron Horse MKIII Team. It's an "all mountain" bike with lots of front and rear travel but lightweight (not like a piggy downhill bike). I ride all the time.
Invest in a decent bike with good components otherwise you will run into BS little things all the time. You don't have to break the bank necessarily like I did but something in the 1.5k - 2k range outta do. Lots of great bike makes out there to choose from. I suggest going and trying a few and get fitted up to something that works for you.
Have fun and don't forget your helmet!