first-what body type are you, long legs/short torso or short legs long torso or somewhere in the middle.
second- get out and try as many bikes as you can!!
Third, look at websites for something you like, there are so many brands to choose from so don't get stuck. kona, trek, giant, ironhorse, cannondale, specialized, haro, marin, etc etc...check ebay out, they have a huge list of mtn bike brands
fourth-price range, specific components and such you would like, disc brakes?
my favorites have to be the ironhorse mk's, giant reigns, and the cannondale prophet for its versatility. but don't limit yourself, shop around!
Definitely need some more details (as the others have said). Also, it's important to get out and ride as many different models as you can before you make your decision. This will help you find the best fitting bike for your style of riding.
I personally have an '04 C-dale Jekyll and am 5'4 145lbs (pre-pregnancy) and it functions quite well. They have replaced the Jekyll frames with the prophet, which is quite a nice bike. I am also getting a new Yeti 575 for next season (selling the Jekyll) because I want something with a little more travel
I agree with the concept of riding as many different models as you can with the one caveat - fit and setup is crucial to how any bike is going to feel to you. I demo'd Trek Fuels twice, the first time it was awesome - had a great ride and loved the bike. Six months later I rode essentially the same bike, also at a Trek truck demo, and hated the bike - difference was that the shocks were set poorly the second time and the fit was off. Now I am a big believer in getting the bike fit properly. Titus has a good explanation of how Women Specific designs work at http://titusti.com/womenfit.html which opened my eyes to what I originally thought was just a way to sell to women. Actually led me to my local Titus dealer and he was the only person in our area who really listened to me about how I wanted my bike to fit. After talking to him and getting measured I ended up with a small Moto-Lite that seems to fit like a glove. I am 5'3" and 120 lbs, but have a long inseam (31") so until now I have been riding bikes that I was made to fit, rather than bikes made to fit me! Makes a huge difference.
I agree with the general opinion that you should try out a lot of bikes first. Something to also note: Kona is coming out with it's first women specific full-suspension this year (Lisa). Might want to look into that as the specs look pretty good.