Keep a notebook of comedy ideas. Many stand-up comics write their own material because they find it easier to deliver material effectively when they locate the humor themselves. Good jokes come from many sources. Pay attention to life around you and write down observations about anything you can put a humorous twist on. You're not writing prose. You're writing a humorous sketch, so a short synopsis of the joke that covers the basic set-up (the joke) and the pay-off (punchline) will do.
Practice delivery in front of a mirror. A good joke can be ruined by bad delivery, just like excellent delivery can also make a bad joke play well to an audience. A good delivery is all about timing--the rhythm of the joke. There's no specific way to deliver a joke. Once you begin performing, you'll get a feel for the timing that makes a joke work. Watching yourself in the mirror helps you focus on your timing and your facial features. Facial features and hand gestures can play a part in your joke delivery as well.
Perform on amateur night at as many comedy clubs as you can find near you. This is the best way to try out new material and to perfect your routine. It's live and the feedback is instant. The audience will applaud or throw things. You will be able to determine your success accordingly. Performing in nightclubs and comedy clubs is also a good way to make a name for yourself in the comedy world, though you may need to consider moving to more than local clubs. This requires you to decide whether your career is worth traveling or not.
Network with other comedians and consider a booking agent. A booking agent isn't necessary, but having one can help get you access to bigger comedy clubs that pay higher performance fees.