Starting at 14 or 15 is a great time to get going, but to really learn the proper technique you need to train with an accomplished teacher. Even with a dance background you can't learn everything you need from watching tapes.
Look around for a youth dancesport program where you live -- try checking the various studios to see if they have programs to get younger dancers started with dancing and competing. If you live near Los Angeles, New York, or San Francisco you are definitely in luck because there are excellent dance studios with youth programs in all these places. There probably are in other cities, too, you just have to find out what studios are in your area and start calling them up and asking questions.
If you can find a beginners program you don't need a partner. The classes are taught in groups and the people in the classes swap around while they're learning to dance so everyone gets a chance to try things both on their own and with a partner. As you progress, you can see about practicing outside of class with the people you're meeting, and perhaps one of them will "stick" as a partner. Youth dancesport coaches can help you get hooked up with a partner too. And if you don't want to compete, and just like to social dance, then you're really all set because when you go to a social dance everybody asks each other to dance and you don't need a partner then at all.
Really, the first step is to find out what dance studios are within a reasonable drive of where you live and what sorts of programs they offer for youths and for beginners. The best thing would be to find a program dedicated to training people your age, but if there's nothing like that where you live then just find a place you like with beginner classes that you like. When you take group classes, talk to the teachers you meet and other students about what teacher the recommend for one-on-one work. You can take private lessons from a teacher and even dance in competitions with your teacher if there are no youths around to dance with. But the perfect situation, of course, would be to find a dedicated youth partner dancing program.
