in fact, your player must be subject to gravity ( so he will usually fall when no ground is under him ) and jump my disable temporarly gravity
something like this should work :
If Player_Jump > 0
REM If player is actually jumping, we continue the movement.
PlayerYPos = PlayerYpos - JumpStep
Des Player_Jump , 1
Else
REM player isn't jumping. Does he ask for a new jump ?
If player_want_jump = 1 And Player_Jump = 0
Player_Jump = JumpHeight / JumpStep
PlayerYPos = PlayerYpos - JumpStep
Else
Rem IF PLAYER DON'T JUMP SO WE USE GRAVITY HERE...
Endif
Endif
player_want_jump = 1 if keyboard key to ask for jup is pressed.
JumpHeight = height in pixels the player jump at maximum
JumpStep = height we move player each jump step.
I hope it helped a lot