Why does my singing voice give out so fast?

I'm a 14 year old male tenor and I really like to sing, I'm good. I have very little professional training though, but when my voice has given out in the past, I've been that I sing out of my throat, and that I shouldn't. How can I stop my voice from giving out so fast, so I can sing an entire song and are there exercises or techniques that will stop me from singing from my throat (whatever that means).