There are many factors that can cause hives such as foods, medications, and other allergens, as well as other physical factors like heat, cold, emotional stress, exercise, sunlight and water. There are, of course, many others-- this is just some more common causes.

If it's mild then you can get some over the counter medicine like Benadryl or Claritain.

If that doesn't work then speak with your doctor for some prescription medications.

If it is an emergency, then seek medical attention immediately