TTR-125 or YZ85
I'm personally a Yamaha fan so if you're looking for a great beginner bike, I would recommend the TTR-125. Everyone I know who previously owned or still do own one, absolutely love it. They say it has the power without being too intense. If you want to start racing, the YZ85 is great. It shouldn't be too big for you or too small but that's coming from me not knowing your exact height and weight. I currently own a YZ85 and it rocks. It started my racing career. I'm actually trying to sell it so if you're interested, let me know. It's a 2006 with some bad ass graphics. Best of luck! :)