Im not sure which branch to join.?

Im a 13 year old girl and although some people say my mind will change by them [i hope it doesnt] I want to join some branch of the army,navy,marines, etc. but im not sure which. I want to see combat I dont want to sit behind a desk or be a nurse. I dont know which one would be better for women. i really want to join the marines but my brothers are too & they tell me to do air force. I really want to do marines but im not sure. I was hoping that when i get out of marines that i could join the air force but whats the age limit and how long does a marine have to serve? thanks.