For your limited budget, I can't in good conscience recommend a new bike. At that price level, you end up with sub-par components, suspension that doesn't work and a very heavy bike.
Your budget is steering you to "big box stores" for your purchase. Those bikes are one size only, heavy as tanks, and not designed to be ridden off-road (read their warning stickers).
I'd suggest the used market for you. Get a bike one or two years old that was an entry level bike from a reputable manufacturer (Trek, Gary Fisher, Specialized, Giant, Haro, Diamond Back, Fuji, Raleigh, Iron Horse,...etc.) and learn how to fix and adjust your bike. (
www.parktool.com)