How do you increase your full voice range?

I sing tenor in my school choir but I have a limited full voice range. I have a really big falsetto. It's larger than my full voice range (my falsetto is two octaves up starting at middle C). I'm pretty sure I can increase my full voice range significantly, I just don't know how. I don't think I am supporting correctly and it is just easier to sing from my throat, that's why I start my falsetto so low.