Just something to think about before you hit the bike store.....where do you really want to ride your bike? Are you getting a "mountain" bike because you will be riding bike trails and want to be able to occasionally ride the dirt edge or be safe on the shoulder of a road or just because it's more comfortable? If yes to these questions, then check out the hybrids (wide tires, slightly more upright bars in many models, the comfort of a mountain bike, reasonably priced compared to true off-road mountain bikes).
BUT! If you plan to ride some gnarly trails on this particular bike--really rough dirt--then you want a tried-and-true mountain bike and that will put you back a bit more on cost. Why? The frame is going to be put together differently (more robustly with the frame brazing (how it's fastened together), more robust shock features, etc., etc.
A hybrid bike will handle some trails, but you probably want to opt for one with tires that are more in the 2.25" range rather than a hybrid that is more street-bike looking (with tires that are narrower than a true mountain bike, but "chunkier" than). You should be able to find a good bike like this somewhere around $300.
I know I'm getting long here.....with your bike, you should get as absolutely basic: helmet, water bottle & cage (any decent bike store will throw a bottle and cage in for free--ASK), and a pump for the tires (that you can attach to the frame of the bike and take with you). If you're a handy person and think you can change a flat on the road, then get a patch kit and tire irons (to remove the tire). If you're brand new to cycling, you might skip that part because it can take a little practice to not only change the tire, but get the wheel lined up just right when you reattach it. So...in that case, ask the shop to recommend an "instant" solution or hey, go geeky and have them put in Mr. Tuffy tire liners (serious cyclists scoff at these, but since you're not exactly racing and counting every ounce, your big concern is just not getting stranded somewhere with a flat tire, right?
If you can swing it, I recommend a small seat pack. It attaches under the seat and you can put your ID (always carry it!), some change/money, your car keys (when you take your bike to a special place for a ride!), kleenex, and any amount of oddments (mine always has a rescue inhaler in it).
OK...sorry for the long note. I just LOVE talking about cycling (duh).