Game Maker

I am creating a Platformer and need some help with the jumping. For all the different codes I try, I come up with nothing. Can anyone help?

Have you checked this out? platform-game

Yeah and if you want a double jump effect you can use a variable to an effect of:

In the create event:
jumps = however many jumps you want to be able to do
jumpable = jumps

In the jump event:

if jumpable < jumps +1
-do the code for jump-
jumpable -=1

In the collision with ground event:

jumpable = jumps

I used this in a couple of games each with a different means of jumping but in all of them I was able to limit the number of jumps until the player hit the floor.
Be wary, though you have to make the ground object different from say a wall or ceiling. other wise you will have to find out which side of it you are hitting and that can take awhile, I find.

What exactly are you trying to accomplish? Just a simple jump? double jumps? Are you having problems making the character jump or is it having them come down or is it stopping when the character hits the ground?

I'm trying to get a single jump, but the game isn't letting me…

I have it to when you press up, it sets the vertical speed to -3, which worked before, but now, there is no limit to the jumps I can do, which I am trying to limit to 1

Have you read the yoyo game tutorial for platform games? This is covered quite well in the tutorial and there is of course an example of how to jump in the first section of the tutorial.