Need good beginner dirt bike?

I am a big fan of atvs (hence my user name) but I would like to try riding dirt bikes. I am 6 feet tall 250 pounds and 18 years old. I would like to know options on both 4-stroke and 2-stroke bikes (would prefer 2-stroke but I can live 4-stoke). I was mostly looking at 125 2-strokes and 250 4-strokes and will mostly be riding trails but would like a bike with some kick so I can fly around at the sand pits were I do a lot of riding as well. Any suggestions would be greatly appreciated.