Bike sizing help please????????

I have an 18" Trek 4300 and I think its a little small. I have a birthday coming up, and its my 16th so a new bike wouldn't be a bad idea for a present. I love this bike though, and if possible I want to keep it. I am 6'0" with a 36" inseam. When I sit on the bike, my feet are tippytoes on the ground, but when I put my feet on the pedals, they dont go nearly all the way staright when I pedal. Instead, they are still bent. Should I get a new bike? I am still growing, and I have a large bike trip (about 400 miles) with my school coming up soon. HELP?!?!?!