Poor baby. The most important things that you will need to purchase are:
1. A suitable container--I'm a huge fan of using rubbermaid or sterilite bins as JKFish said. They're ideal for sick fish because they're light and super durable for easy cleaning, and they're long and shallow as opposed to narrow and tall. This is important because bettas breathe air from the surface, and when they're sick it can be hard for them to reach to the top to breathe. The shallower the container the more time the fish can spend resting and recovering.
2. You will need a dechlorinating product--city water is treated with chlorine and chloramine to make it safe for drinking, but these are chemicals that will poison fish. I recommend Seachem Prime, it is concentrated so it lasts longer than other bottles and you get the most real product for your money. Believe it or not, using tap water and a dechlorinating product is much better than spring water without.
3. You will need a heater. Bettas are tropical fish, they need stable temperatures around 78-83 degrees to be comfortable, healthy, and active. Cold blooded animals rely on heat from outside sources to drive their entire metabolism, so if a betta is kept at room temperature it may have serious longterm consequences. I recommend a 25 watt adjustable heater--pre-set heaters, mini-heaters, and heater pads don't work on a thermostat, so they will overheat the water or not heat it enough. They are a waste of money. Here is an example of the type of heater you should purchase: http://www.fosterandsmithaquatics.co...m?pcatid=11368
4. If you can find it, Methylene blue is a very gentle medication that can help prevent infections and parasite infestation while your fish is in its vulnerable state. Try to find a product that is pure methylene blue without any other additives like salt/electrolyes, because if you want to add more aquarium salt later, you don't risk overdosage. If the store does not carry methylene blue specifically, don't let any of those "helpful" pet store employees convince you to purchase other medications--many employees are quite misinformed and tend to pass out medications willy-nilly with no idea of their consequences. It's best just not to listen to them at all 95% of the time.
The most important thing is to get him into clean water and keep it clean. First, put the fish into a temporary container, like a plastic cup with some of his tank water in it, of course, while you're doing the water change. Make sure the cup is away from hazards like open drains, other pets, etc, and put something over it so that he doesn't jump out. Pour out all the old water, make sure that you have something to catch the gravel or other items in the tank--nothing ruins a garbage disposal faster than runaway gravel. Rinse out everything in the tank and the tank itself with hot tap water to remove any uneaten food, feces, and ammonia residue. Next, put the gravel and other items back in the tank and fill the tank with water from the tap that matches the temperature of the fish's old tank water. It's a good idea to keep a small sample of the water near the sink, so you can dip your hand back and forth between the old water and the faucet until you can't feel any difference in temperature. Then add the dechlorinator, if you purchased Prime brand dechlorinator, you will only need a tiny bit. Give the water a good stir, and then float the container you put the fish into in the tank. Every few minutes, pour out some of the water in the fish's cup, and add a small amount of the new water. Do this in small increments over the course of about 15-20 minutes if the water is similar to the old (same source, same additives, same temp). If the water is different in any of these components, spread this process out over about 30-45 minutes.
Very small containers demand a lot of water changes--to give you an idea of how often you will need to change the water, containers of less than a gallon need to be changed in the way I described every day, one gallon containers will need a change every other day, two gallons will need a change every 2-3 days, five gallons will need one change once a week. Containers of 3 gallons or larger can undergo a process called the nitrogen cycle with the help of a filter. During this process, the filter pad and other surfaces in the tank become colonized by specialized bacteria that break down the toxic ammonia that fish excrete as waste into a much less toxic substance called nitrate--once this process is complete, you only need to do one partial change per week instead of frequent 100% water changes. This is how most people choose to set up and maintain their fish tanks.