If I get stung by a jellyfish what is the best treatment?

I think we will bust a commonly held myth with the answer to your question.

Several years ago a TV show recommended urine after a jellyfish sting. In reality, urine could make the sting situation worse. Jellyfish, those bulbous Medusa-like creatures, float near many of the world's beaches. Some of the jellyfish's skin cells are stinging cells, or cnidocytes. These specialized cells have organelles called nematocysts that contain venom. Cnidocytes are spread along the entire length of the jellyfish's tentacles.

Once stung, angry, red, whiplike lash marks appear on the skin. The pain radiates from the sting site and starts to itch, burn and throb as it blisters. Scratching it, though, can make the pain worse, because rubbing activates the nematocysts, which release more venom. The best treatment is to rinse off the nematocysts with salt water. Urine may be too dilute and not contain enough salts to be effective in getting rid of the stingy darts, in fact, dilute urine may cause the nematocysts to fire causing even more pain.

Source: http://www.scientificamerican.com/article.cfm?id=fact-or-fiction-urinating

Can you tell me more about jellyfish?

Jellyfish are interesting animals but not ones that you would want to get real close to.
During their life cycle, jellyfish experience an alternation of generations in which one generation (the medussa) reproduces sexually and the next generation (the polyp) reproduces asexually. The medusa form is the dominant and most recognized form of the jellyfish.
In the first part of a jellyfish's life it is a stationary polp and is attached to a substrate. A polyp is cylindrical and stalk-like in form. At its base is a disc that adheres to the substrate and its top is a mouth opening surrounded by small tentacles. The polyp feeds by drawing food into its mouth. It grows and begins to bud new polyps from its trunk. As it does, the polyp develops into what is called a polyp hydroid colony. Members of the polyp colony are linked together by feeding tubes. The entire polyp hydroid colony, like the originating polyp, is sessile. The polyp colony can grow for several years. When polyps within the colony reach an adequate size, they are ready to begin the next stage in the jellyfish life cycle - the adult medusa.


Source: http://animals.about.com/od/cnidarians/a/lifecyclejellyf.htm