Pay attention to people who seem to be out of place when you're in public. A nun in a bar, or a punk rocker in church seem strange, yet amusing. You can find such contradictions in the world by paying attention. Go to the library where everyone is supposed to be quiet, and listen to the loudest person. Watch others' reactions to this person. This could be material for a comic sketch or a part of your routine. Contradictions provide humor.
Watch comedy actors, and take notes about what makes them funny. Steve Martin in "The Jerk" created a character so dim-witted and unaware, he was hilarious. Mike Myers created dozens of hilarious characters throughout the years, such as Wayne in "Wayne's World" and several characters in the "Austin Powers" films. Pay attention to the characterization. How is the actor using his body and voice to create comedy? Make notes and try some of these characters yourself. Then create your own characters and place them in a situation that seems contradictory. Write everything down so you don't forget your ideas.
Write material that excites you and comes from an honest emotional place. Jerry Seinfeld based his comedy on observing the world and saying what honestly annoyed him. Most audiences have thought the very same things in their day-to-day lives and found his humor funny because they agreed with him. Truth is funny. Observe and record things in life that annoy you. Perhaps it's an old lady in front of you at the ATM or the size of soda cups at fast food restaurants. Write about what honestly annoys you. Odds are, someone else has felt the same way.
Find an improvisation class in your area and take some classes. Improvisation is a tool for actors and comedians to help unlock creative energy. It's also helpful when you're stuck with writer's block. Follow the instructor's advice in the class and make notes when something inspires you. By getting on your feet, you have a better sense of what performing your own material feels like. Allow your partners to affect you. You might find something you never thought of before.
Call your local bars and comedy clubs to find out when they have open mic nights. Once you have material ready and present it to an audience, the funny material becomes very obvious. Have fun performing,and don't expect to sound like a professional comedian on your first few tries. Make notes about what worked and what didn't work, and use this information as you write new material.