First, have your instruction appear on screen using a string and Text.
I am assuming your camera is already sitting in the car.
Do the tutorial on strings ant text for your instructions.

http://tutorial.3dgamestudio.net/
(workshop #7) if you haven't already.
and tell the string to say your instructions.

Then have your car run an action called controls.
(you can call it whatever you want) and use the following code:

action controls()
{
while(1)
{
if(key_m)
{
//do whatever you need it to do
}
wait(1);
}

with this code you will have text on the screen saying "press the M key to turn on the car"
and whatever you in put the part where I said "do whatever you want"
will happen when they press the M key.

making the text go away when they press the M key is a bit more complicated
so try and get this working, when you are done reply to this saying
you're done and we can move on to the next steps.
Hope that helps