First of all, you should take shower and make sure the water is not hot at all as it will aggravate the pain. Fresh water is best for shower especially when you have got sunburn.
You can also add peppermint tea in your bath tub which is a reliable remedy for easing the pain of sunburn. Make sure you do not use regular soap after getting sunburn as it can make your skin dry that will not help in healing the affected area of your skin.
Take aloe vera gel and paste it over the sunburned area. The affected area should be covered fully and it will help in reducing the pain in a little time.
You can also use wet compresses to the sunburned area which will help in relieving pain from the affected area of your skin.
After getting your skin sunburned, you should take ibuprofen or aspirin at least three times a day which will help in reducing the inflammation and pain.
If you have got sunburn on your arm then try to wear half-sleeve shirt which will not make you uneasy. If you have got sunburn on any other area of your body like back or neck then try to use loose clothes in order to avoid itching.
Sunburn usually causes blisters and you should avoid breaking these blisters to release the water. It can cause further pain as the water can spread blisters around the existing blisters. Use antibacterial ointment which will let the blisters break automatically.
After getting sunburned, you should increase the use of drinking water which will help you avoiding dehydration that can aggravate the problem.
You should also use moisturiser on the sunburned area which will help you in removing the effects of sunburn and your skin will get its original colour.