I have been having sex for two years now and I can't seem to get pregnant, I want to if it because I am 19 years old and body is not fully developed? I missed my period a month and got it the month after and the month after what is wrong with my body?

Their may not be anything wrong with your body. Do you know how to tell if you are ovulating? Were you on birth control at any point? Are you taking prenatal vitamins? Do you have any poor health (anemia, etc.) that you know of?
Its also possible that your partner has some fertility problems.

If you've actually been trying for 2 years, its time for both of you to visit a doctor.