you could have a y_velocity# variable for the player, and set the y_velocity# to some positive number whenever the user presses the spacebar or whatever your jump key is, and then decrease the velocity by 9.8 meters per second, and then set the y velocity to 0 when the character is back on the ground. You'd need to update the player by the velocity. Plus only "jump" when the character is on the ground, or else the y velocity would always be positive whenever the spacebar is presses, giving the player a jet-pack effect.
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Specs: Sony VAIO Laptop, Windows XP, P4 2.8Ghz, 512MB RAM, ATI Radeon 64MB video memory, DBP Upgrade 5.3.