A jellyfish sting can cause neurogenic shock, an allergic reaction, and a strong intoxication of the body. Here’s what you need to do to avoid or relieve the unpleasant consequences of such a meeting:

Immediately clean the wound, and get rid of the remains of the tentacles. To avoid a new sting, don’t touch tentacles with your bare hands.
Wash the sting area with salt water. Never ever use fresh water for this: it activates the stinging cells.
To remove toxins, make a compress of apple cider vinegar or alcohol.
Drink plenty of water, and visit a doctor. Water will help you recover faster, and the doctor will prescribe the necessary medicines.