As you are undoubtedly looking for a serious answer I will try to help. Firstly it must be an idea that you find funny. If you don't find it amusing when you think of it others are unlikely to.
It is often easier to think of a situation or punchline then work backwards and write the story that sets up the joke.
Keep the whole thing as short as possible, don't try to pad it out with uneccessary detail.
The best jokes work by having an unexpected end often by getting the reader to deliberately interpret one word in a particular way then twisting it's meaning at the end. A recent example being:
Heather Mills says she is going to buy a plane with the divorce money.
But she's still going to use Immac on the other leg.
The misinterpretation of the word plane is the key to the joke.
Good luck.