Bettas are normally very easy to care for, and they have so much personality! Here are a few little tips I've picked up on caring for bettas. I am by no means an expert! This is just what's worked for my bettas and me.
You may also want to look around to see how the other fish are faring. If you see tanks with lots of sick or dead fish, if the area is dirty, or you see a lot of empty betta cups (indicating thrown-away dead fish), you may want to reconsider purchasing a betta there.
Water should be changed frequently--at least once a week if there is no filtration. I started out using bottled water (store brand water is very cheap) because it had no chemicals or algae spores. Eventually, I started treating tap water to use. I neutralize the chlorine with Stress Coat and add half a teaspoon of aquarium salt per gallon. The Stress Coat protects your betta from injury in addition to treating the water, and the salt prevents infections.
Be sure your betta is in a warm area away from direct light and drafts. Try to place him where he can see around him (bettas are so curious!) but avoid a high-traffic area where his bowl might be tipped over.
Whichever food you buy, look for a few key ingredients to maintain health and improve color. Good quality foods should contain fish meal as a primary ingredient, brewer's yeast, carotene, biotin, a form of cobalt (probably cobalt carbonate), a form of calcium (probably calcium pantothenate), and some sort of fish oil. Higher quality foods will also contain shrimp meal, wheat or corn meal, folic acid, and Vitamin C (probably in ascorbic acid form).
For snacks, I give my fish Tetra Blood Worms. The worms are a little big, so I tear each one in half first. They love them! Sometimes, I also give them Tetra Baby Shrimp, which I crush into tiny flakes.
If your betta isn't eating much, or is coughing up clear, stringy material, he may have an internal fungal infection. Sluggish behavior may indicate a bacterial infection. Torn fins or grey areas on the edges of fins usually indicate fin rot. I treat all of these maladies with MelaFix, made by Aquarium Pharmaceuticals. Its active ingredient is tea tree oil, a natural anti-bacterial, anti-fungal remedy that won't harm plants or other aquarium life. (Not even baby fish!) I add 1/8 teaspoon per gallon, once a day for three days. I think tea tree oil is like catnip for fish, because my bettas will swim right up to the spot where I added the MelaFix and dance around in it. LOL! Aquarium Pharmaceuticals also makes Spledid Betta, another tea tree oil remedy specifically for bettas, which is much cheaper than MelaFix.
Other illnesses include dropsy, which causes the scales to stick out like on a pinecone, and ich, which causes tiny white cysts all over the body. Check petcare sites or ask an expert on how to treat these diseases.
Bettas are usually happiest living alone. A betta can safely live in a community tank as long as there are no fin-nippers or other bettas around. This usually isn't a happy arrangement for the betta, though, and should be avoided.
If you want a roommate for your betta, there are a few animals he should be able to get along with well. Otocinclus and plecostomus can live peacefully with bettas, and also keep a tank algae-free. Both need aerated water to survive. Corydoras can clean up scraps of food and other debris, but like to swim in groups of three or more, so they should only be used in larger tanks.
Snails and ghost shrimp also live peacefully with bettas, and can clean up debris. Be sure your shrimp has plenty of iodine in his diet, preferably from dried baby shrimp. Snails should be used with caution!! Snails often have parasites, like hookworms, which can infect your fish. Keep snails in quarantine for at least a month before moving them into your tank. Another possible tankmate is the African frog, which are fun to watch. ^_^
Whichever you choose, be sure to watch for agressive behaviors or illness in one of your fish. Animals will naturally attack a weakened tankmate, and any sick fish should be quarantined to avoid spreading infection and to allow time to heal. All animals should be kept in quarantine for at least two weeks before introducing them to your betta's tank (four weeks for a snail). Please do not skip the quarantine step!
Male bettas have a fighting instinct, which can be fulfilled without putting them in any danger. They will flare at their reflections, or anything remotely betta-shaped. Try pushing two betta bowls together so they can flare at each other, or put a mirror next to your betta's bowl so he can flare at his reflection. Just don't leave them too long, or you'll wear your bettas out! My bettas always seem happier after having a good "fight," and they often build bubble nests the next day.