i was just wondering if , when you become a pilot, wether its AF, Army, Marines or Navy, do you get to choose what type of fighter jet or helicopter you want to fly?

Depends on (1) The needs of the Branch, and (2)Where you rank in your class at the aviators course of the Branch. You can put in a "Dream sheet" for what you want in the Naval Aviators course. I can't speak for the other two branches. Back to the class rankings, the higher the class ranking, the more likely you get your choice of A/C. But the needs come first.

You're right about that. I finished in the top ten of my class, had jets on my dream sheet and was assigned helos. Best decision the Navy ever made for me :]

It's the same way in the Army, we pick based on class order of merit. For us it was half check ride scores and half PT test.