Introduction
The first step to learning how to take care of a Betta fish properly is to understand them. As an experienced Betta owner, I can tell you a couple of things: First of all, Betta fish are finicky, they like things a certain way, and if you can't get things just right, they start having trouble. That's not to say Bettas are a bad pet, however, because the other thing I can tell you is that Bettas are extremely rewarding and interactive pets, and by reading this you're already taking a positive step in setting yourself up for success. 
Betta fish originated in Siam, or modern day Thailand, several hundred years ago and were bred as fighting fish. With dull colors, stubby fins, and a bad attitude they hardly resembled the pets we have today. However, after decades of breeding we were able to develop these beautiful fish in the early to mid 1900s, and they quickly became a household favorite. Betta fish show remarkable personalities and behaviors, such as sticking to the side of a tank to watch you, jumping up out of the water when it's meal time, excitedly dancing about, and even the ability to learn simple tricks just to name a few. But before you get to the fun stuff, you have to know how to keep them alive, so let's get started!
Supplies
Betta fish don't demand a lot, and the nice thing is that most of their needs are a one-time purchase, meaning you'll likely never need to spend more money than your initial investment.The supplies you'll want to have on hand before you bring your Betta home are...
- A tank (we'll discuss size/type in a minute)
- Food (pellets or flakes are both fine)
- An aquarium heater
- A small aquarium filter
- An aquarium thermometer
- Rocks for the tank bottom (this is called "substrate" in the aquarium world).
- De-chlorination drops for your water
Beyond these items, Betta fish care doesn't take much. The only exception could be treatment if your fish falls ill, but these are self administered, only a few dollars, and likely won't be needed anyways, so don't worry about it for the time being.
Tank Selection & Setup
"How Big of a Tank do I Need?": Whether you have a tank you want to use already or haven't gotten one yet, I'm going to clear up one of the biggest arguments amongst Betta owners so that you get started off on the right foot. The debate I'm talking about is that of tank size, and many people would have you believe that the cute little cubes and bowls that pet stores sell specifically for Bettas are a suitable living space for your finned friend. The truth however, as any Betta care expert will tell you, is that the rule of thumb is that each Betta needs roughly 5 gallons of tank space, or as close to it as you can get. If you can go up to 10, awesome, my most recent Betta loves his ten gallon tank, but you should at least find space for a 3-5 gallon, if you only have room for a half gallon cube, you need to make space for a bigger tank before you think about caring for Betta fish.The confusion behind tank size comes from the face that Betta fish in the wild lived in the ponds of rice paddies in farming and irrigation fields, which some people interpret to mean puddles. However, the truth of it is that while these ponds were shallow, they could span tens or even hundreds of feet, and Bettas are clearly more happy and active when they have ample room to swim around in, trust me on this one you'll enjoy your pet a lot more if he's happy, and he'll let you know as well!
Tank Setup: Alright, so now that you have your tank, the first step is add the rocks to the bottom, attach your filter (the type that hangs over the tank's edge is easiest), stick your thermometer on the side, and attach your heater. Now fill the tank with water, leaving about 2 inches of space between the top of the water and the top of your tank so that your Betta won't jump out. Next, add de-chlorination drops (refer to the bottle to see how many drops you should add per gallon of water, then multiply that by your tank size in gallons), and let the water sit for an hour or so.
Water Quality
Temperature: Before you add your Betta fish, the first step is to get the right water temperature. Caring for Betta fish requires that you maintain a water temperature between 78 and 82 degrees Fahrenheit. If the temperature drops too low, Bettas become sluggish as are at serious health risk below 70-72 degrees. Consistency is also as important in temperature, meaning once you get into a proper temp. don't fiddle with your heater as lots of changes in how warm the water is can stress your fish out. Generally, this is pretty easy to achieve and you can adjust your heater, wait a half an hour, and see where your thermometer is. Once you're in that 78-82 zone (or as close as possible), you can add your fish!Water Changes: After you've put your Betta into the tank and everything is running smoothly, you'll want to do water changes on a weekly basis... but don't worry: this is super easy! In fact, you don't even need to clean the tank or remove all of the water, in fact removing too much water is a bad thing, because beneficial bacteria cultures will start to grow in your tank that are necessary to keeping ammonia and nitrate levels down.
My recommendation is to remove 20-30% of your water and replace it with fresh (de-chlorinated!) water just once per week. This takes just a couple of minutes, keeps your water quality balanced, and when you only do these medium water changes once per week you don't stress out your fish as much as doing smaller changes more often, or a huge change every once in a while (this is one aspect of Betta fish care that usually isn't explained to new owners)..
Feeding Betta Fish
When it comes to Betta fish care, feeding is one of the simplest aspects, though it often gets messed up. First of all, the brine shrimp and various live or freeze dried foods should be used as snacks, and not as a daily diet; they are high in fat and lack other essential nutrients.This will basically mean you'll buy either pellets or flakes to feed your pet, and they should be made for Betta fish (i.e. don't use gold fish food for them). Either pellet or flake form of food is fine, and while it is important to read the instructions is is not always the best to follow them. Overfeeding is a common danger for your Betta fish and the best way to avoid it is to feed Bettas just once per day, and probably about half the food bottle's recommended amount. They have extremely efficient metabolisms, and under-feeding is not nearly as much of a danger as overfeeding for Bettas.
Betta Tricks & Training: Betta fish food is also a great incentive when training your pet to do tricks. Yes, you heard me right, Betta can be trained to do all sorts of things for food from jumping out of the water and grabbing food from your fingertip, to swimming through a hoop, etc. This takes a while and it's important to let your Betta get settled in his new home for a couple weeks before trying this, but a trained fish is sure to impress friends & guests!