A limerick is a poetic form that can be particularly fun to read and to write. Limericks are often humorous, meanspirited, or pornographic. I'll explain the form, and you can decide how down and dirty you want to get. A limerick doesn't have to be bawdy to be successful, but a good limerick is often appreciated for being irreverent and often transgressive.
The point is to violate taboo and walk to the edge of social convention. Learn From The Masters. As with any genre of writing, you can best learn from its masters. Write line 2 and 5 to rhyme with the first line. Now write lines 3 and 4 with a different rhyme.
When you are done writing, read your limerick out loud to see if it has the right rhythm; three beats on lines 1, 2, and 5, and two beats on lines 3 and 4, as shown above. If not, see if you can rewrite some words to get the rhythm right. How can the answer be improved? The limerick is a very structured 5 line poem. Originally, according to Wikipedia, limericks were a bit obscene, but that has changed with the times. The first, second and fifth lines, the longer lines, rhyme.