What do I do for sudden swelling of toes of one foot?

Ice. Ice now and make sure you follow up with a podiatrist. If you have fever, nausea, vomiting or diarrhea you may have an infection. If you have not have any trauma to your feet this could be a possibility. You will need antibiotics. Other causes could be insect bites, stress fractures or capsulitis. So please see a podiatrist as soon as possible.
Visit a doctor. To find the cause. Toes don't just suddenly swell! particularly in one foot. Examine closely, look for a possible bug bite, look for any interruption in skin from maybe a fungus etc...A podiatrist will be able to help.