interactive Vehicles

I dont npw howb to create a inreractive vehicle 4 a fps gem plz halp!!!; Ok, i want develope a Low-Poly FPS Game, but i dont have any idea how to create interactive vehicles. I would use the BIK plugin for animation. There should be a little animation and then changes the player the character from solider to the tank,car,… (sry 4 bad en) But i dont now how to do this. And it should work with MP.