No disrespect intended mxprincess, but unless this young lady wants to ride/race motocross a 2 stroke 80/85 is a very bad choice for a beginner. You didn't mention your height which is more of a factor than weight so it is really hard to say what size bike you need. Your best two choices will be something in the Yamaha TT-R line or Honda CRF-F line. With the TT-R there is a 125 with a 14" rear wheel and one with a 16" rear wheel, try both of those on for size at the dealer. In the Honda lineup try the CRF100F and CRF150F, there is also a CRF80F and a CRF230F it the others are too large or small. You really cannot go wrong with any of these bikes. Sit on all of them, decide which you like and then if you don't want to spend the bucks on a new one look on cycletrader.com or craigslist for used ones.