Scratch

That's a pretty widespread question. Make it.

Well sart with maybe a car...
Forever; if space bar pressed; move 2 steps
Do you get that? Basically start off with some controls for a car, as in forward, back turning left, turning right.
Check out my game called Racer===. Search it!
Then download it, click on one of the car scripts and just see what I have made.

The best thing to start doing is make a remix! That is when you take someones project, then add your own little touches, this will teach you the basics.