You have three options on the fan:
1.) Don't use it, seems to be the most popular.
2.) Buy a balancing kit.
3.) Follow these simple instructions:
Turn the fan off (I'm not getting sued for any injuries here)
Make sure that all of the screws holding the blades to the fan are tight, this is the most common problem and will fix 2/3 of wobbly fans.
If it's a 4 blade fan it's pretty easy to determine the next part. Turn one blade toward you and look to see if it is in the same position as the one opposite it, repeat with the next blade. If they don't line up you have a bent blade and can attempt to straighten it, but you might break it at this point. If you break it you'll need all new blades (they're manufactured at the same time and balanced in the factory) and your best bet at this point is to buy a new fan.
If it's a 5 blade fan it gets a little trickier on the bent part and considering the cost of most new fans, just buy one.
If it's still wobbling and the blades appear stable you can go to the hardware store and purchase the following:
Packing tape
Flat washers 3/8 works
Clothespins
Set the ceiling fan to blow down, (counter clockwise). Clip a CP (clothes pin) half way out on the lower edge of any blade. We will call this blade #1. If the wobble problem was real bad, clip two or three next to each other in this same location. Now run the fan at the highest speed possible. If the wobble starts getting real bad before reaching high speed, turn the ceiling fan off. Now move the clip or clips to the next blade, blade #2 and repeat the step. Do the same for each blade. Observe the amount of wobble as each blade is tried. You will notice that one way will be better than all the rest. On the top of this blade, you will tape a 3/8 flat washer approximately where the clip was. Note: The blade surface must be clean to the point where the tape will stick adequately. Otherwise you may sling a weight off while the ceiling fan is running and this can be dangerous. Test the surface by sticking a piece of tape to the area and see if it comes off. Now stick a piece, approx. 3" long, on the top of the blade in this area. Then using another 3" long piece, tape the washer down on top of the first piece. Note: you will use one washer for each clip that was on the blade. Run the ceiling fan. If still wobbling, put the clip back on the blade where you put the first weight, run and observe. Move the clip to the blade on the right of the weighted blade. Run and observe. Move the clip to the blade on the left of the weighted blade. Run and observe. One of these three will be better than the others. Put a weight on top of the blade at this point. Repeat process until the wobble is stopped. Normally 1-3 weights will be sufficient, but if real bad it may take 6-7. If the wobble gets worse with the CP than without it, then you have too much weight on already.