What a coincedence, I'm in a class that dived headfirst into GameMaker 7. So I have experience with that. But my first attempt at a platformer failed miserably, with the character floating on air half the time.
So hey, if you need help on the basics of GameMaker, I can help you out as well. Still, going through tutorials and re-creating basic programming concepts (preventing an object from moving off the screen, using a loop for drawing an object over and over, approximate collision detection using the distance between the centers of two objects and checking against the sums of the radii of two imaginary circles around the objects) is a good start to learning GameMaker. Oh, and pick up "The GameMaker's Apprentice" from some bookstore, that's a huge help.