Just a topic for me.
DO NOT GET A BIKE FROM WALMART!!!!!!!
Go down to your local bike shop and have a look at the lower end Trek, Specialized, Fuji and so on.
Specialized has a good model called Hardrock. They can be had for ~$300 if you shop around.
Trek 4200 is another good start. Get a bike with a 8 speed system. Stay away from 7 speed an lower.
Make sure you get at least Shimano Alivio components, preferrably Shimano Deore but it seems like the same model of a bike changes from year to year and in general the manufactureers tend to 'down grade' when a new model year comes out.
For example I got my Specializeed Hardrock in '01. It came with mostly Deore components. The next year they kept the price the same but went down to Alivio.
This is the ladder of Shimano components, top to bottom. I might have missed the really lowest end.
Acera
Alivio
Deore
Deore LX
Deore XT
XTR
Look at last years bikes on clearance. Check out
mtbr.com and read some reviews. It help a lot.