The best and easiest way to discipline a cat is with a spray bottle of water. Just spritz the cat when you find him or her doing something you disapprove of and they will run off. That and loud noises work really well. You can put some coins in a tin can and shake it anytime you catch kitty doing something naughty and they will eventually learn to not do that anymore.
A water gun is the best way. And it's not mean. It might take a couple of times.....but it really works. Just remember though you Cant squirt him/her for mp reason. Be sure I catch em in the act. Good luck!!
You don't say what your cat is doing wrong. Not using litterbox? Is box clean? Is cat fixed? Scratching up furniture? Cats have an innate need to scratch - get a scratching post. Consider WHY your cat is acting out before punishing.