Well, I don't know what code you've been given, but this is usually how I have a jumping system:
Movement is based on 'velocity', so instead of always using the Move Object command, I continually use the Position Object, and keep offsetting the current position by a given velocity, which is calculated during the main loop.
To make it jump, all you have to do is check two things: 1) You press the appropriate button, i.e., space bar, and 2) you are currently colliding with the ground. If both these statements are true, then simply add an arbitrary value to the Y velocity (it should only be added during one frame of the loop).
Finally, while there is nothing directly beneath the player, continuously subtract a given gravity value from the Y velocity.
