Don't. Seriously. By all means, keep a knife on you just in case. But never ever use it as your main weapon. It's so short, you're basically hugging the zombie, but do you really need me to tell you that?
If you need to fight off Zombies, you've come to the right place. Here we'll talk about different weapons to keep those pesky Walkers off your tail, with the least amount of hassle for your own safety. But as long as the Zombie Apocalypse™ hasn't started, don't bother about arming yourself, training with the different weapons, and all that other depressing stuff. Go watch some videos on YouTube instead.