While you're on your way home to get further treatment, hold your arm in an elevated position to help minimize the swelling. Gently wash the area to remove dirt and any other debris from the area. This will help prevent an infection. This will help to alleviate the itching by bringing down the swelling and numbing the stung area. Cycle the compress in 20-minute intervals. Apply it for 20 minutes, then let your skin recover for at least 20 minutes. You can also use an ice pack. Just be sure to have a barrier like a cloth or paper towels between the ice and your skin. These over-the-counter medicines will help alleviate the pain and itching. After a few hours, the swelling will subside a little and a blister will form. As long as you don't pop the blister, the area shouldn't become infected. Avoid scratching it, as this can cause it to break.  If the blister breaks, cleanse the area with soapy water and monitor it for signs of infection. You can also apply an antibiotic ointment to any open stings. These ointments are available over-the-counter from most drug stores and pharmacies.  If the area becomes discolored or starts leaking pus, it may have become infected. Seek medical attention immediately.

Summary:
Elevate the affected area. Bathe the sting with soapy water. Place a cool compress on the infected area. Take an antihistamine or use a hydrocortisone cream. Don't pop the blister.