What's a good beginner dirt bike for a guy my size?

Well, I would like to get into dirt bikes. I’m wondering how much bike I should get. I’m 5’5, 14 and growing. I’m probably somewhere around 140 lbs, but I’m not sure as we don’t have a scale in our house. I can ride my friends Kawasaki 250 4 stroke pretty well, but other then that I have no experience. I’m looking for something under $1000 preferably. I do have access to a atv/mx store, which is within 15 miles of my house. So the question is, what can i get? I have no problem with buying a used bike and I’m willing to work on it instead of sending it to a shop.