How do I stop it from overheating?

You need more data for a correct answer. Start with the simple things: if the fan starts/is the thermostat stuck on close? More complex problems would be a blown headgasket, broken waterpump, etc.

If it's an original radiator you may want to upgrade that , if it's old it may be somewhat clogged

Could be several things. First, does it overheat while city driving but not on the highway? If that's the case, it's probably your fan clutch or belt. Second, how long does it take to warm up? If it warms up and overheats around 5 minutes (while at idle) then it could be your thermostat (easy to replace) is either bad or installed backwards (it happens). Third, is there water flowing in the radiator after it's all warmed up? (you need to start it without the cap on and let it warm up. At first flow should be low/non-existent and then start flowing pretty freely. If not, it's probably the water pump - easy way to check is to remove the belt and turn it by hand - if it's REALLY easy to turn it's bad, if it spins but feels kind of thick, that's normal - it's pushing water. Also, check the hoses to make sure they are firm - if they squeeze to easy, they could be getting suction blockage (when suction is hard enough to make the hose go flat) I suggest flushing the radiator out and while you're at it, remove the thermostat and put in a new one (they are fairly cheap and usually first thing to fail anyway). You could pressure test it as well to check for leaks - they sell kits at most auto parts stores.