Don't buy tank too big for them, they will get bore quickly. How much are we calling too big? Many bettas enjoy being in huge tanks, as long as they are densely planted. My largest tank is only 16 gallons (at the moment, anyway), but some people keep happy bettas in tanks as large as 75 gallons.
Don't put them in a community tank, other fish will rip his pretty fins and he wont last. Again, not true. Some fish are very nippy will chew on fins. Other times the bettas will attack community tankmates. However, a community with a mellow betta and non-nippy fish, such as cories, ember tetras, kuhli loaches or male Endlers is very possible.
Don't use small tank, he will die very quick or will grow very slowly. Again, not entirely true. Anything under a gallon is definitely too small, and although I would never do it myself, many people on here have made 1 gallon tanks that are perfect homes for bettas. Some bettas have swimming issues or tail-bite in large spaces and actually need smaller tanks.
Doesn't matter the type betta you have, the good size from my view will be 5g tank. This I agree with 100%. I love my 5 gallon.
Plastic or live plants works. Only use plastic if it is a soft plastic with no sharp edges. If it snags pantyhose, it will snag a betta tail. Silk is generally better, but live is best.
For plants, i use mostly green cabomba, they are easy to maintain and grow fast. All my 5g tanks have that plant. Cabomba is great, but check your local laws - it is illegal in some US states. Here in Australia it is also illegal.
Best betta food is live blood worms, adult brine shrimp or mosquito larves (if available). If you dont have live food then use Atison's betta food. You can buy Atison products at IBC website. Other good foods include New Life Spectrum and Hikari.
Water change, do as much as you can. Some say, do only half but i empty the tank and put new water. Even half, your tank still dirty and have bacteria that you cant see with your eyes. Depends on the tank size. In a 5 gallon tank with a filter, you tank will probably complete the nitrogen cycle, and overcleaning will cause the cycle to be set back. In a 5 gallon with one betta, 25-50% a week is absolutely fine, especially with live plants. Make sure you use a gravel vacuum, though. Dirt you can see is generally not harmful. The harmful stuff (ammonia, nitrites and nitrates) is invisible. With a tank under 5 gallons with no filter or live plants, do one 50% and one 100% change a week.
Gravel, dont use the ones that have coloration like blue, red, green or other colors. Use the natural ones, none coloration. Again, it depends on the gravel. I'm not a fan of coloured gravel, but usually it is fine. Some can leach colour into your water though, so test before putting it in with fish.
Exercising your betta once a day for few minutes. Either put a little piece of glace in front of his tank or if you have females betta, put a female in a transparent plastic cup and put in his tank. That will stress your female out unnecessarily. Just use a mirror.
If possible, dont use filter or bubble, betta dont like their water moving too much, they get nervous or a lot of stress. If you want to use it then get sponge filter and something to control the air pump. In a 5 gallon or over, you actually do want a filter - this will allow the tank to cycle and save you a lot of work. Sponge filters are great, I love them, but other, stronger filters can be baffled so the flow is very gentle. I have the very powerful Fluvals U1 and U2, but baffled they present no problem for my fish.
Try not to catch your betta with a net when you do water change, betta hate nets. If you have betta scoop use that if not then use your hands. I use my hand to catch them. If you do partial changes, you can leave your fish in the tank - much less stressful. If you have to do a full change, ladles are better than your hands. You can accidentally rub their slime coat off if you use your hands. This is bad.
If your betta fish get sick, use indian almond leaves. You wont find those leaves in your local pet store, only online, look for aquabid or IBC. +1, I love these things. You can use them when your fish isn't sick, too. They harden scales and make bubble nests stickier. They also tan the water.
Those advices are from me. Other members may use differents way to keep their betta healthy and grow old. You'll decide which way will feet you better. Good luck