Read THIC's Wikipedia site. Urine is a BAD idea and reducing sting (and removing tentacle) depends on species. A Man of War is not a jellyfish, for example, but does have a fierce sting.
vinegar causes the nematosist to solidify therefore the liquid inside of them doesn't irritate " sting" you. Pour it gently so it doesn't rupture the cell. The solidified cells easily then will rinse off with the vinigar. This is your best answer.
Peeing on the sting or try pouring vinegar on it. Then seek medical attention. You did not state location. Some jellyfish are more venemous than others. Do not ingest anything. He was just joking. Connect with me directly via Twitter @ link
The Mods. . . . then switch to another website for a brief rest, and allow the sting to subside, and the Mods to take care of The Stinger. If it is still smarting after that, I find a good stiff drink is just the ticket. . . . .for, ahem, Medicinal Purposes, of course.