I want to know about what i should be doing for when im older in the army?

I want to join the army, i wana be part of the i guess tanker crews. I wanna be on the battleiled though i dont wanna be back 6oo miles from the battle fixing tanks. Is there something else i might be interested in ? Im in 8 th grade. I wanna know what i should be doing. Should i be getting fit? What gradeds should i be getting ? Theres this thing i can go to in 10 grade . I can do automechanics and stuff like that , what that be a smart option so i get to know how those types of things work ?