a yamaha ttr 125 is a good first bike for riding around on not to rough ground, but if you were looking for a first bike that when you got good enough you could race. i recomend a yz 85, they are quick when you want them to be but they can also be pretty smooth and not to hard to ride. my first bike was a ttr 125 and i found that after a week or 2 it felt not powerful enough.
hope this advice helps )