What Spiffy said is right on.
Let's dig a little deeper though and see if we can find out why your Prelude is overheating.
You've done the most logical thing, to check the coolant levels. Are they normal? At the top of the radiator tank and at the full, middle or bottom mark on the overflow reservoir?
If they were low and adding coolant stopped your overheating, then problem solved. Flush the system and the radiator and you're good to go.
Still overheating....then it could be a couple of things. Gotta know how it's overheating to make the right general diagnosis.
If the engine overheats at slower speeds, like in traffic, but then cools off at once you start moving at higher speeds, you're looking at a problem with the cooling fans. You can check to see if your fan is working by letting the car idle while watching the temperature gauge. Make sure that your AC is turned off.
After letting the car idle to temp, with the AC OFF, you should hear the cooling fan come on somewhere between 1/2 and 3/4 of the gauge travel. If it has not come on by 3/4 gauge, then something is wrong with the fan system. If it's not too far away, you can probably drive to the shop instead of towing, but avoid stop and go traffic and watch the gauge carefully.
If it gets into the red and stays there, pull over, let it cool off before you even think about opening the cap, add water and try and drive a little further to make it to the shop. Just remember, even sustained high temps for short periods can seriously damage your engine. BE CAREFUL when making the decision to drive on.
If the engine overheats at high speeds, that usually is indicative of a problem with coolant flow. Flow can be reduced by a partially plugged radiator or a thermostat that does not open all the way (a thermostat that does not open at all usually causes the car to overheat all the time). You can do one of two things here, first you could remove the radiator --
(very easy to do -- drain the coolant via the radiator drain valve, remove the lower and upper hoses (and ATF cooler hoses if it's an auto trans) unplug the fan connectors, remove the radiator brackets at the top, and lift the radiator out, then remove the fan shroud and assembly) --
Then take the removed radiator to a shop and have it flow tested, OR, you can remove and replace the thermostat and see if it cures the problem. It's usually cheaper to replace the thermostat than to test the radiator, and the thermo is where I would start first.
Good luck in figuring it out and if you have any other questions about this or anything else, just ask.
If you want to try and do some or all of this yourself, here are some links to the online manuals -
http://media.honda.co.uk/car/owner/medi ... 0/5-62.pdf
Refilling and bleeding
http://media.honda.co.uk/car/owner/medi ... 0/5-64.pdf
Cap and radiator testing
http://media.honda.co.uk/car/owner/medi ... 0/5-65.pdf
http://media.honda.co.uk/car/owner/medi ... 0/5-66.pdf