Join a mma gym, if you're good enough your coaches will set you up in a few amateur fights to see how you deal with fight pressure.
If you do well, perhaps you can have a future in it.
Right now, you are a very young person, so that dream of yours may very well happen if you work hard and if you have the natural talent inherent in those who aspire to reach UFC.
Since you are a boxer right now, focus on that, takedown defense and BJJ defense to your game to start with.
Watch (over and over) as many fights of mma fighters that utilize that kind of approach (JDS, Rampage, etc) and absorb as many knowledge possible while doing it.
Meanwhile if you want to guarantee a future for yourself and your family to be, make sure you give your self a proper education. If you don't have the brains or money to make trough superior education, do a professional course of some kind (don't know how they're called there in the US) to make sure you give yourself a profession. Not having a profession and looking for random jobs that pay shit is really a nasty way of living, believe me you don't want that for yourself, i've been there so i know. Took me a few years to learn that.
Anyway, you're 15 yo. You are very young and it is perfectly possible that you achieve your goal of becoming a UFC fighter. I wish i had discovered mma at 15 instead of discovering it at 28, i'm currently 33.
Good luck pal