First place a piston pointing where you want the car to go then place a sticky piston facing the other way then the regular piston. Place slime blocks in front of both pistons. Now place an observer with the red stone output facing down at the sticky piston. Next place a slime block behind the observer. To turn it on place a redstone block on the piston and the car will go forward.

(From now on its just extras to add to your car) to add a seat place a rail next to the obsever(on the lower slime block) and put a minecart to sit in, keep in mind the rail will break when the car starts.

If you want a flame thrower fill in the missing space with a slime block(if you still want your seat put the rail on top of it). Place 2 observers side to side in frot of the car with the redstone out put facing out. Then place 2 dispensers facing forward and fill it with fire charges. When you turn the car on it starts to shoot. You can also fill up the dispensers with TNT or arrows pretty much anything works.