Emir,
You and I live in the same area, so I can share with you what I know.
As far as I know (and I'm pretty sure), there are three legit BJJ blackbelts teaching in the Central Connecticut Area (Danilo Chernan, Rafeal Formiga, and Fabiano Teles).
Danilo teaches at the Lion's den. This may be a good place for a 13 year old guy because the cost is relatively low (about $80 per month) and you can have unlimited classes in BJJ, MMA, and boxing as well as access to the free weights gym. Could be a good way of exposing yourself to everything and seeing what you like.
Fabiano teaches at Underdog. I know of a few people that have had some success with amateur MMA, and then decided to go to Underdog to improve. If you know for sure that you want to focus on MMA, then this is as good a place as any. If MMA were my focus, I think I'd go here.
If distance or price is an issue .......
I know that in the last few weeks, a guy has been rolling at our gym, he is a purple belt from a Matt Serra school in New Jersey. He moved up here to start his own business. He has just recently opened an MMA gym in Bristol, across the street from ESPN. I don't know the name, he was talking about it after class just yesterday and handing out business cards, but I didn't take one.
I don't know if his school will be good or not, but I can tell you that his BJJ is solid. This school should be very close to your house, so this might be a good option if you live on the Bristol side of Southington, because you could get there without having to ask for a ride.
Good luck.
If your only consideration is to go to the school that will give you the best instruction in BJJ and MMA, then I'd advise Underdog. Of course, that is just my personal opinion based on what I've gathered in conversations with people in the area that are involved in BJJ and MMA. I only do BJJ and it is as a hobby that I enjoy.