Consider your audience if you are writing for others. Different age groups and segments of society may find different topics to be humorous. What makes a preschooler laugh will not make an adult laugh.
Write a story song if you are unsure what to write about. A story song does just what its title suggests--it tells a story. Think about a funny incident that happened to you or to someone you know. Real life offers the funniest ideas of all. A song that starts out with something like, "When he walked out the front door, he saw the goat in his car..." might capture a reader's interest and leave him wanting to hear more.
Write the truth. Humor often comes in the form of true statements that many people are afraid to say out loud. A lyric like, "The little things got on her nerves, like the rising and falling of his chest..." might offer a funny insight into what many unhappily married people feel but never talk about.
Use irony and sarcasm. These literary devices convey humor in subtle ways. Instead of saying, "Talking with her was difficult," you could use sarcasm by writing, "Having a good conversation with her was like trying to nail jello to a tree...."
Set your lyrics to a simple melody. Simple melodies usually appeal to the widest range of people.
Write a simple, humorous "hook" for the song. The hook is the phrase that is repeated most throughout the song, usually in the chorus. For instance, if you're writing a humorous song about drinking, you could use a hook like, "Bring me a shot and make it a triple."