this is really the wrong place to ask this and you are wayyy off topic, i will not go into it but its really simple, all you need is a variable like:

var In_Vehicle;

then say in your game, if In_Vehicle == 0, you can move your player, if In_Vehicle == 1 then your in a vehicle, just hide your player, make him passable and snap him to the vehicles position then when you press the button to exit the vehicles, you give control back to the player

i wont go in detail on this... but it should be easy