The best jokes and stories take us by surprise; they lead up to a punchline (called a 'twist') that we just don't see coming. To write a good story or joke, try writing the ending first, then go back and fill in the details leading up to the twist.

Some other tips for writing good jokes and stories:

• be as brief as you can without losing detail
• omit any characters that don't take an active part in the story. If Uncle Harold was there, but didn't take part, then Uncle Harold doesn't have to be mentioned.
• if something has to be explained for the story to be funny (e.g. how to bait a fishing hook and why it matters), do your explaining early on in the story, not at the punchline. All we should have to do at the punchline is laugh.
• use vivid verbs. Compare: "he got into the water" with "he flung himself into the water". See how the second version gives us a much sharper mental picture?

For more comprehensive tips on joke and story writing, see this series of story writing tips. Although the articles are written with a focus on writing memoirs, Parts 3-7 are highly useful in writing jokes and stories.