The previous answer that was here was (dangerously) incorrect - alcohol is *extremely* toxic to cats. For example, a tablespoon (approximately 15ml, less than a single shot) of whiskey is enough to kill the average cat.
The same way you would for a human. Wash the injured area with a disinfectant, make sure the wound is not deep enough to require stitches. If so, a trip to the vet may be advised. If not, apply...
depends
does it have a cut
is its leg swollen
ect.
if it is hurt witha cut wrap it up and take it to the vet but if it is swelling definetly take your cat to the vet it my have a blood...