Standing on a stepladder, grasp the fan body and try to shake it. If it moves easily, the screws that hold the fan to the junction box may be loose.
While a helper supports the fan, remove the screws from the canopy. The fan will now be free from the ceiling, so your helper must hold it securely.
Tighten the screws that hold the ceiling plate to the junction box.
Reinstall the canopy.
Attach a clothespin to the edge of a blade, about halfway from the end. Start the fan on slow speed and check to see if the wobble disappears. If it doesn't, move the clothespin to another blade and repeat. Continue until you've tested all the blades. If the fan stops wobbling at any point, the last blade tested is unbalanced.
Balance the problem blade by attaching a weight to the middle of the top edge, about halfway from the end of the blade. You can buy a weight kit, or use epoxy to glue one or more coins to the blade. Experiment to determine how much weight you need to add.
Remove the blades one at a time, using a screwdriver and, if necessary, adjustable pliers to hold any retaining nuts. Lay each blade on a flat surface. If a blade won't lie flat, it is warped.
Obtain a replacement from the manufacturer or a dealer (take the old blade with you so you can get one that's the same length and weight) and install it.