I have a sprained left foot, in the area of the third metatarsal. i want to know what i can do and how long should i wait to exercise.

I would recommend : You see a doctor. It is a little unusual to sprain a foot over an individual metatarsal and I would advise an xray to ensure you did not fracture the third metatarsal.

Rest, ice, compress: The standard treatment for a sprain is to rest from aggravating activity, ice, compress,and elevate to reduce swelling. You should use crutches and keep weight off of it until bearing weight is no longer painful. Ice at least twice/day for 15-20 minutes to help decrease pain & swelling. Compress with a wrap, etc. Elevate it above the level of your heart (i.e. lay flat w/foot up). NSAIDs for pain.