That's awesome. I'm a little surprised that you got a 250 2-stroke for your first bike. That thing can get you into trouble real quick. A slip of the wrist while falling in that powerband will have you on the ground before you know what happened. That thing is going to be a wild bore trying to tame it on the trails too. But I'm sure you will still have tons of fun. I didn't start riding dirt bikes until I was 20, and it was every bit as great as I dreamed since I was little.
P.S. Especially since you got a beast of a bike, wear ALL the gear, ALL the time!!!!!! Who cares what you look like. Knee guards are a must. At least get a roost guard, but I would highly recommend a full safety jacket/pressure suit. The shoulder, arm, chest, and back protection help so much. You will be glad you did after your first good fall.