I started off on a KX125 when I was your age and had zero riding experience. Two-stroke bikes like this may be too peppy for you, depending on your size. If you're less than 5' 9" you may want a lower bike as the mx'ers sit quite high for clearing big jumps for mx racers.
It's hard to go wrong with the four-stroke Japanese trail bikes like the KL100s and all the Honda bikes.
