At 5'9 tall and being 14 years old, you're going to be doing some growing, quickly. If you decide to go with the TTR, go with the TTR125L at the very least. It's the taller version. My wife has one, she's 5'6, and it's a little short for her at the moment although we're trying to raise it a couple of inches.
The thing is, if you pick up a good running bike for right around $1000, you can always resell it next year if you outgrow it and not lose much if any money. So if you bought a TTR125L and only got one season out of it, you could turn around next year and sell it to buy a larger bike.
A TTR225 might be a better fit for you now and the next couple of years. It's an easy to ride 4 stroke that will do well in the trails, although it's a little heavier than the 125's. I've got a garage full of Yamahas and have had nothing but good luck with them.
If you're truly as new to riding as you say, just be careful of not getting too much bike. Get a bike you're comfortable with, even if it means going with the smaller 125 first. And as I mentioned, if you buy a 3 or 5 year old bike, which you can pick up for around a grand, then you can't go wrong.
Good luck.