Angela - It all depends on what you think you might want to do with your bike. If you have any inklings of triathlon in the back of your head, skip the mountain bike and go for a road bike. A mountain bike is best if you plan on riding trails. If you want a more all around kind of bike, then look at hybrids (you can put road or mountain tires on more of a mountain frame), but bear in mind you probably wouldn't want this for a road race/tri/du. I'm pretty happy with my road bike. I can race with it, and it also is great for commuting in to work as well. Next year I'm hoping to do a 3 day charity ride!
Once you know what your intended use is going to be, then check out your local bike shop and they can show you what they have in your price range. Try them out and see what fits you the best. Bike fit is such a personal thing, just like running shoes. Also, it will take you a couple rides to get used to your saddle. Do not despair!
Good luck!