There are two methods to do this.
TURN OFF THE POWER!
Remove the outlet from the box. There will be one white, probably one wire bare of insulation, and probably one black and one red wire. The black and red wire go to two screws on one side of the outlet - one is switched and one is not. In most, but not all, cases it is the red wire that is switched. Remove the red wire from the outlet and put a wire nut on it, capping it off, and push it back into the box - it will not be used. Remove the black wire and splice it with two more short pieces of wire, preferably black - terminate the wires on the two screws where the black and red wires were.
The other method is to simply replace the outlet - they are not expensive and if old it probably needs replacement anyway. In this case, remove all the wires and toss the outlet. To wire the new outlet, terminate the white wire on the silver screw, the bare wire on the green ground screw, and the black wire on the brass colored screw. Again, cap the red wire and push it into the box, unused.
If you use either of these methods and find that the entire outlet is now switched, you have put the wrong wire to the outlet. Another possible problem is that the current outlet is entirely switched, instead of only half of it being switched. If that is the case, there may not be a constant hot wire in the box: should that be the case, remove the switch instead, remove both wires from it and splice them together with a wire nut (excluding the bare ground wire of course). This method will also work with a half-hot outlet.
Either way, re-mount the outlet onto the box. If you have any problems, shoot me an email - my address can be found on my profile page at: http://hubpages.com/@wilderness
where it gives a link to contact me.