As an addition to Jembeth's excellent writeup above, I thought I'd throw in my two cents, since I very recently sprained my ankle when I slipped on the stairs in my apartment.
The pain was surprisingly intense at first; I was very afraid that I'd broken my ankle and that I'd have to make a trip to the emergency room.
It's always better to be safe than sorry when it comes to your own health, but some of us in the States don't have great health insurance and we are discouraged from seeking emergency treatment due to its potentially very high cost. For instance, my housemate's recent late-night visit to a local emergency room due to a tendinitis flare-up -- the ER was the only after-hours prompt care available in the city -- ended up costing him $500 for a 2-minute doctor's examination. This isn't the sort of money you want to have to pay for something that you can competently deal with on your own.
So, the following is a rough guideline of things to do and watch for once you've injured your ankle. (ObDisclaimer: I'm not a doctor, nor a licensed medical professional, but all this worked in my situation. Use this information, which I believe to be accurate, at your own risk.)
Immediately after your injury:
- Get up slowly and carefully if you've fallen; try to get someone to help you up. Carefully see if you can put any weight on the ankle.
- If you experience sharp, shooting pain upon applying any weight to the ankle, it is very likely broken. See a physician ASAP.
- If the ankle holds some/all of your body weight without causing you significantly more pain, take as much weight off the leg as you can and hobble to the nearest bed/chair/bench/couch and get off your feet. Elevate the injured leg, preferably above your heart, so sitting back in a recliner or lying on a bed is best. If you don't have that option, try to get the leg above your waist using pillows, stacked books, etc. The key here is to keep it raised as much as possible (without causing additional discomfort, of course) so as to minimize swelling and inflammation.
- Ice the ankle for twenty minutes. Don't put ice directly on your skin; put a towel or washcloth in between the ice and your flesh so you don't get frostbite or otherwise damage your tissues. Plastic bags of frozen peas or corn work well, as of course do the athletic gel-based cold packs, which remain flexible so you can wrap them around the joint.
- After about twenty minutes, take the ice off your ankle and compare it to your other ankle. Chances are, it will be swollen, but if it is badly swollen (for instance, if it's twice the size of your uninjured joint) you should seek medical attention very soon. Your ankle will likely also be discolored, but if the bruising is very dark or black, that's a clear sign of a broken bone. If the discoloration is a light blue-gray or nonexistent, you most likely have a sprain.
- If it seems to be a sprain, continue to ice and elevate the ankle. You don't want to ice it continuously, or you could cause tissue damage. A good rule of thumb is twenty minutes on, ten minutes off, twenty on, etc.
- Take some ibuprofen or other anti-inflammatory painkiller as soon as you can. This will help reduce the swelling. However, take whatever medicine you usually take; if you've been taking Tylenol, don't switch to Advil because mixing the two can cause liver problems in some people. And if you've been drinking alcohol, consult a pharmacist for safe medicines to take.
- Get a wrap/support for your ankle like an Ace bandage, an "air" cast (which is a plastic padded splint with velcro straps), or a neoprene ankle wrap. The key here is to stabilize the ankle and to apply some compression to keep the swelling down. Don't wrap it so tightly that it hurts or cuts off your circulation.
The first 48 hours after your injury:
The first two days are crucial (and painful). The important thing is to keep your ankle wrapped, elevated, and iced as often as is feasible. Stay off your feet as much as you can. This means that it helps to have a sympathetic roommate or a significant other to help you out. You absolutely, positively don't want to do anything to injure the torn/stretched ligaments in your foot any further. A severe sprain that involves a lot of torn ligaments can be harder to recover from than a broken ankle.
If you have to walk someplace, get a cane or a crutch and make sure your ankle is well-protected and stabilized. But your best option is to camp out on your couch or bed with your wrapped, iced ankle propped up on some pillows. Plan to catch up on your reading or video watching; don't plan to go out unless you absolutely have to. When you sleep, keep the ankle wrapped and the leg elevated on a pillow or two at the foot of your bed.
If you have a sympathetic boyfriend or girlfriend around, by all means take advantage of the situation. Chances are you can get them to run errands for you to help you rest your ankle. And once they see you lying helpless on the couch, all wrapped up with no place to go, they'll almost certainly want to "cheer you up". Repeatedly. It's all good; just don't jostle the ankle.
By the second day, the chances are good that you'll start having a mild to severe backache if you're at all prone to back problems. This backache can be caused by leaning to the side as you walk mostly on only one leg, by an odd sleeping position, or by jarring to your body during the original injury. The ibuprofen/naproxen/whatever you take will help with this, too, but an electric massage mat can also come in handy.
After the second day:
Your ankle should be feeling much better after the first 48-60 hours. You can probably walk around without assistance, but move carefully and keep the ankle wrapped. I found that wearing a wrap under a pair of sturdy, low-heeled boots with a gripping sole provides good ankle protection.
Your ankle will be swollen, and it will turn interesting colors. You may see bruising on the sides or bottom of your foot. The swelling may move from the front of your foot to the back, then vice versa depending on how you're keeping your foot wrapped.
Over the course of the first week after you sprain your ankle, it's going to feel uncomfortable aside from the expectable pain. Little muscles in your foot and lower leg will cramp randomly. You may get a tingling sensation like ants crawling under your skin or soda pop bubbles moving through your flesh.
If you're still feeling a lot of pain, seek medical attention as soon as possible.
Don't ice your ankle after the first 48 hours (unless you re-injure yourself) as it won't do any more good against swelling and could impede healing at this point. After the first couple of days, you might find warm soaks in a tub to be soothing, particularly if you've got lingering back pain.
It will probably be several weeks before you're back up to speed, depending on the severity of the sprain. Athletes should not do any heavy training for six weeks after a sprain. The ligaments around your ankle will still be loose and vulnerable, so keep it wrapped for up to three weeks. If your ankle seems stiff, do gentle streching exercises as Jembeth suggests.
Also, be sure you're getting adequate protein and nutrients in your diet, or you won't heal up well. This is important for anyone, since it's easy to want to just huddle miserably on the couch eating Cheetos if you're in pain and can't walk. But watching one's nutrition may be especially important for vegans and vegetarians who may be working from a borderline protein deficit to start with.