Just go check it out. DO NOT go alone,as you're only 14. Just go take a free bjj class, as many offer this for free. See if you like it. Bring an adult with you, and it should be fun, and friendly, and you should be shown around, and explained the finer points of what they train there. No pressure. Ask questions about the instructors, they should be HAPPY to answer you. it's pretty easy to tell by talking to the instructors if it's a good school, this may not be obvious to you as you are young, that's why I'm saying bring an adult. Even before taking a class, go watch one !
Be weary of any place that tries to make you sign up for a long period of time right off the bat. Be weary of any place where the instructions brags how good he is, or doesn't make you feel welcome and helping in answering all your questions with zero pressure.
Sorry for long winded post, but there are so many terrible martial arts schools. I'd go just for the Ribiero Seminar !!!!!!