It can be a bad sprain. Rest prevents further injury and avoids stress on already inflamed tissue.
Put the ankle joint at rest by wearing a brace or splint.
Ice is probably the best treatment.
Ice will counteract the increased blood flow to the injured area.
It will reduce the swelling, redness, and warmth.
Applied soon after the injury, ice will prevent much of the inflammation from happening.
Compression can help keep down swelling. Compression wraps, such as ace bandages, do not provide much support to prevent movement of the ankle, and you should not apply them tightly. Elevation (keeping the injured area up as high as possible) will help the body absorb fluid that has leaked into the tissue.
Ideally, prop the ankle up so that it is above the level of your heart.
You can do this in a reclining chair.