If you are overheating, you may notice it on your temperature gauge, or the bike might be getting unusually hot, or you may have problems running. This is actually a pretty serious problem, you don't want to blow a head gasket or crack/warp your head or block. Its best that you address this problem as soon as you can. Here are some common causes.
The most common is simply low on coolant. Take a look in your reservoir and see if there is any extra coolant. If not, you may be low, but not necessarily. Once the bike has cooled back down, open up your radiator cap (when it is cool to the touch) and see if there is coolant in there. If not, then fill it up, and see if that solves your problem. Do a further assessment for leaks to explain why you were low in the first place.
If need be, it is possible to open the radiator cap with the bike hot. Do this at your own risk! The danger is that hot, pressurized coolant may spurt out at you. However I have done it many times without injury. The trick is to utilize a safety feature built into the caps. First, get some heavy gloves, rags, towels, whatever you need to do to protect yourself from hot steam. Then, rotate the cap just one click. This should be enough to start releasing steam. Stand back and let it release. Then when it is done, you can open it one more click and open the cap and look inside. Practice this on your bike when cold to see how it works.
If you are overheating, you may have a stuck thermostat. These fail from time to time, preventing the circulation of hot coolant. The repair is replacement. If your thermostat is gunked up and stuck, so too might be your radiator, which can also cause overheating. A good flush and replacement of parts will help you out.
You may also have a failed water pump. These actually aren't that hard to replace, and can be repaired in an emergency scenario. Here's how to do it: