Your 15T will be a fine gun. It was my first AR also, but mine was the older one that came with the Troy goodies. They were quite the deal then.
Check out Primary Arms for a decent budget optic. If you go with a budget optic, make sure to get one with a QD so you can get it off and use your iron sights if you have to.
Lots of people are going to tell you to save up a little more for a better optic, and they're right. You can buy a used Aimpoint or Eotech (battery life or reticle, pick one) for not much more than $200. I think I bought a used Aimpoint for around $300, with a special mount. I bought a used Eotech XPS2-2 for not much more. Just watch the EE like a hawk; the good deals will go fast.
Here is what I did with it when I first got it. Cheapo red dot.
Here is how it looked before I cannibalized it for parts.