There are always five lines; no more, no less. Sentences 1, 2 and 5 correspond with each other and are of the same (phonetic) length. Sentences 3 and 4 also correspond with each other and are short. The last sentence should contain a punchline (i.e. funny remark, moral of the story, joke).
And don't worry: I'm not very good at Limericks either.