What is a good beginner dirtbike?

Stay away from the Chinese crap and stick with a major company like Honda, Suzuki, etc. Get a 4 stroke, it'll be easier to learn on. At 5'3", you won't be able to fit a full sized bike yet, so look for something around 150 or so. Honda makes a CRF100 that's supposed to be the ultimate starter bike, and if you go older to the XR100 or somewhere around there, you'll get one in your price range easy. Avoid MX bikes for now too, they take money to keep running and the power hit can put you on your head if you're not experienced. Also helps alot to have an experienced rider to teach you. If you don't know anyone, check meetup.com for a local riding club, someone there would be willing to teach you the basics. Don't forget gear either, you can get a good kit that includes everything from boots to helmet from chaparral racing for about $300 bucks. I'm serious, don't skimp on the gear, you will fall off, it's half the fun, but no fun if you break a knee cap or scramble your eggs.

A pitbike is just that, a pit bike, and not suitable for using as a dirt bike. If you get a pit bike you will soon loose interest in it...