Scratch

I take it you've got a sprite for the ground. Basically you want to have a script like this:

Forever
     if not touching sprite [ ]
           change y by [ 1 ]
End

That will make it so that whenever the sprite isn't touching the ground sprite, it will fall by 1. You just need to make it so that when the sprite jumps, its y increase is higher than 1.

Hope that helped 

Thank you!