No, you don't need to add live aquarium plants for a betta fish. Although I do recommend getting atleast a fake aquarium plant that is a bit bushy (probably a non-snagging variety because most betta's have long fins that could tear/rip) and reaches the top of the aquariums waterline because betta's do seem to like resting on top plant leaves (they may even sleep partially out of the water at times and are capable of doing so unlike most fish since betta are 'labyrinth breathers'). Without this type of plant, your betta will probably spend a good amount of time resting at the bottom of the tank. The best fake plant I have found for a betta is the Exo Terra Jungle Plant Amapallo which you can latch onto the top of your aquarium.
Do I need aquarium ornaments or filtration for my betta?
Some betta seem to like aquarium decor that has some type of cave/opening, but it's important a betta can get in and out of it easily or it may get stuck.
I suggest using a slow flowing filter system for betta fish (betta don't do well in turbulent water conditions), I recommend the Marina S10 Power Filter for small betta setups or a sponge filter for bigger setups (as these can take up a lot of room in smaller tanks). These fish tend to be more active when there is a little water movement at the surface, especially during feeding time as I have found these fish to be very visual and prefer food when it's moving at the surface. A filter also helps provide some surface agitation which I find is needed as betta can excrete a mucus-like film on top the waters surface, which can affect the water's oxygen level.
How often should I do water changes for my betta?
Water changes depend on how often you want to clean your tank (I recommend testing your aquarium water to help you better determine that). Betta's are generally not messy fish, but if you have a slow cycling filter (as you don't want a powerful turbulent filter for a betta) then once or twice a month is probably suffice. Keep in mind that betta's are sensitive to rapid temperature changes and it could cause them to get things like 'ich', so be sure the new water you put in the tank is within 4F or less of its current temperature.
How much light does my betta need?
Aquarium lighting is optional, although in a room with natural window lighting coming in it's probably suffice enough (just don't keep the tank directly in front of a window or under air ducts). Some people suggest that lighting can help a betta's coloring, but I don't really notice changes in the ones I've kept personally. If your betta is in a live planted tank then you may need enough lighting to sustain the plant rather than the fish.
How much do I feed my betta, and what do I feed it?
As for betta food I recommend small floating betta pellets (flakes often come out their gills when fed it, and betta aren't typically substrate foragers so sinking pellets aren't a good option for them). I highly recommend using New Life Spectrum's Betta Formula (feed betta's as much as they can consume in about 3 minutes time, or when they begin spitting out and/or avoiding food), this is great for betta's that are picky eaters. As an occasional treat, you can feed your betta a tiny bit of bloodworms, I like Hikari FD Blood Worms (I have not had much success with other freeze-dried varieties) or frozen bloodworms which are probably the best option. Betta's do seem to have a diet that requires a high amount of protein (which may be why they're a bit on the aggressive side).
Are there any water additives my betta habitat requires?
If your tap water contains chlorine than you'll need to add a water de-chlorinator into the water before adding your betta into the tank or new tap water. I don't recommend using aquarium salt just because there is a lot of online talk stating that it's not good for labyrinth breathers like betta's, and it may cause them to leave more of a mucus-like film at the water's surface. I personally like NutraFin Betta Plus Water Conditioner because it contains 'tropical almond leaf extract' and almond leaves are supposed to be beneficial to them. Betta in the wild tend to live in 'blackwater' conditions and thus almond leaves help to recreate that natural environment for them (you could also try using Tetra's Blackwater Extract). Male betta may even create 'bubble nests' under dried almond leaves (or at the waters surface). The NutraFin product does not help with ammonia so you may want to use something like Hikari Ultimate to help combat that (which is also water de-chlorinator). If your betta does get sick with 'ich' or a white fungus I do have another post on trying to remedy that.
Does my betta need an aquarium heater?
If your house is kept around 70°F indoors than your betta is probably fine without an aquarium heater. If your betta does get sick some people say having an aquarium heater may help, more so when trying to treat 'fin rot' (although I'm not making any guarantees on that). An aquarium thermometer is optional but may be a good idea if you're a beginner, or if you want to know the water temperature of your aquarium water at any given time. I do feel betta are a bit sensitive to dramatic temperature changes, and I actually prefer not using an aquarium heater for my betta since I like having my water at room temperature in a separate container/bucket for 24-48 hours. Just like with aquarium lighting people also say that a heater can help betta coloration (however, I have not really noticed any significant changes with one and some small betta heaters I've found can cause more harm to bettas than good).
Does my betta require a substrate?
In my opinion it's not totally necessary, but if you have a tank that gives off a lot of light reflection I would recommend it because reflective surfaces in a betta habitat can cause the betta to think his reflection is another betta fish and this causes male betta's to act a bit aggressive (you'll probably see his gills puff out and he'll possibly attack things if this is the case). I also wouldn't recommend marble/glass substrate either, just some basic solid colored gravel sand or pea gravel in the color of your choosing (I prefer the sand gravel). If you're thinking about using aquarium gems as your substrate than take caution using those as some betta like resting on the bottom of their habitat and it could injure them.
Can I have multiple betta fish living together?
Betta fish are also known as 'Siamese Fighting Fish' due to their aggressive nature with one another. Male fish should not be kept with other males as they can harm one another, resulting in death. I have read that male betta fish may be kept with at least two female fish, but I have not personally tried that out. I have also heard that betta fish that were bred and raised with other betta fish for most of their lives may be able to co-habitat together. You may be able to keep female betta fish together, but again I have not attempted that for myself. You can put a betta with some small non-aggressive varieties of tropical fish (I have been at a fish store when I overheard a lady mention her betta did not get along with her guppies), but you may want to avoid colorful fish varieties as well (in general do plenty of research before attempting to put a betta with other tank mates).
What size habitat does a betta fish require?
I find this is probably the most sensitive subject when it comes to keeping betta fish, especially when you see them being sold in tiny cups with very little water at pet stores. Those tiny cups are definitely not ideal for a betta to live out the remainder of its life, it is a very sad situation and a lot of betta fish will come with some amount of health problems (fin rot is probably the most common problem). Honestly, I have found that an adult betta could probably get away with living in something smaller than a gallon of water. Betta aren't overly active compared to a lot of other fish so a lot of water really doesn't seem to be required for their survival, but this does not necessarily mean that it is great for their overall health. I would probably just suggest that for every 1-inch length of a betta, they should have 1 to 2 gallons of water. The betta I have are generally kept in a 2.5 gallon and overall the single betta's I've kept in it seem happy and active. If you plan to try and breed a pair of betta fish then you will probably want 5-20 gallons to use just for breeding purposes. A lot of people seem to suggest the minimum tank size for one betta is 5-10 gallons but the more water you can provide any fish is obviously going to be more beneficial. From my experience most adult sized betta generally live around 2 years (although there are apparent records showing betta have lived 9-10 years, and the highest age I've seen mentioned is 15 years).
Personally, I feel a 10-gallon tank is an optimal size for a single betta fish and this is simply based on viewing the fishes overall swim pattern in a larger tank. I do feel you could probably add in a 2nd betta to this 10-gallon tank if they are compatible and you don't have to add in a divider because adding a divider really defeats the point of the fish having the adequate space to move freely in that a 10-gallon non-divided tank can provide for it.
Aside from aggression do betta fish have other personality characteristics?
Betta can have their own unique personalities in general, some of which can be more aggressive, playful, lazy, shy, active, etc... and some may be all of the above at times. I also feel like they're a fish that can also get depressed, which I feel is generally due to their habitat. If you take a betta from a larger tank and move it to a smaller tank, the fish can suddenly become less active. Even if you change a betta's decor I feel it can affect their mood and activity level as well.