I agree with Dave (or guardrail as we know him as on the road/trail) for most of what he says, but I do feel the need to put my two cents in. I also feel like now would be a good time to mention that Dave has WAY more MTB experience than I do. I have never been in a MTB race (yet) and only have about a year of trail riding experience under my belt. However, I do have a lot of experience with different components from working at the shop. That being said, most of what both of us have to say is really only pertinent if you are planning on doing some serious riding. If you just want to get to class and back, it helps to have a nice bike, but just about anything will do (although we will laugh at you if you roll up on a Wal-Mart special)
Brands: mostly doesn't matter. Pretty much any bike shop brand is going to be of good quality. The biggest thing to pay attention to is the geometry that a particular manufacturer uses. There all about the same, but there can be little things that make one frame superior to another. For instance, if you look at the top tube on a Giant mountain frame, there is what looks like a divit in it. This one thing helps a lot with frame clearance and standover height. Also, what kind of tire clearance does the frame have? Can you easily fit a 2.5 inch tire? It always sucks when you find out that your chainstays are too narrow to fit those fatties that you've been dreaming of.
Frame material: I'm pretty much opposite of Dave here, I think steel is the way to go. Although steel does rust easily, if you properly care for the frame, chances are that it will last you longer than an aluminum or carbon frame. As long as you treat the inside of the frame with frame saver, and make sure to cover any external scratches with nail polish or touch up paint, your steel will last a long time. Steel can also take a beating, if you dent that aluminum or carbon frame, chances are that the frame is finished, chances are also high that a steel frame wouldn't have been dented in the first place.
Breaks: Dave is pretty much right on. I LOVE V-breaks. Discs are nice too, but don't get anything other than avids (bb5's or bb7's), the adjustment capabilities of the avid caliper are way superior to any other offerings out there, I could explain it, but it would take a while. If you go disc, go mechanical. Hydro is way too much cost and maintenance. Is easy to replace a cable, it sucks to replace a hydraulic line.
Gear Stuff: Shimano is awesome. the deore (LX) line is relatively inexpensive and works beautifully. But I also think that SRAM has some very strong products when it comes to their rear derailur and shifters. The shimano and SRAM rear derailurs are very differently engineered. The spring mechanisim on the shimano derailur will allow it to bounce up and down when you hit a bump, whereas the SRAM derailur is locked in place vertically, which is nice because nobody likes to listen to their derailur bounce around. The SRAM shifter (not the gripshift one) works really awesome, although I think the finger positions are a bit awkward, and I wish it had a bit more "clikiness" or a better feel when you shift. The shimano shifter is sweet too, not too much more to say about that. I'm not the biggest fan of the SRAM front derailur, but it works, and actually, I pretty much hate all front derailurs.
Suspension: Some people like it. I don't. I feel like I have much better handling and control on a hardtail bike with a rigid fork. Not to mention the extremely high cost of overhauling a suspension fork or the shock in your rear linkage.
Helmet: get one that you like, it's the one that you're most likely to wear. One hand adjustment makes life a million times easier.
Locks: U-Lock or high quality chain (from a bike shop, not a hardware store) if you want to keep your bike. Kryptonite makes the best (in my opinion)