Gary Fisher Mamba about $300, decent entry level bike!
Haro ICS 2.0 about the same price, likewise a decent entry level bike.
Stay away from Box Store Bikes. KleinMp99 suggested a great website for beginners check out
www.bikesrnottoys.com it's got answers to all you questions and to some you haven't thought of!
L8R
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger