You need to make sure it's just a sprained ankle and not something more serious like a broken bone or torn ligament.
If it's only a sprained ankle, try to keep weight off it. As much as possible, don't walk or stand. Elevate your foot and ankle.
Initially, keep it cool. Apply ice (not directly on the skin, wrap it in a towel or bag), and not for longer than 20 minutes at a time.
Take anti-inflamatories and painkillers (aspirin or ibuprofen) using standard cautions (see labels).
If it's really just a sprain, it will heal with time.
Like with all medical issues, take advice from the Internet with caution. Check with trained medical personnel as necessary.