I know it is ok to keep betta in under 10 gal tank in US, but I will tell my version anyway.
I would recommend at least 10gal tank for betta. That is because I think there is not enough impulses for fish in tiny tank. In this size tank you are able to put filter and water will stay cleaner. Biological filtration is better because of oxygen. Clean water also prevent deseases.
With 10gal you can do water changes for example weekly, in smaller you would probably need to change water more often. Live plants are preferable, like in any tank ofcourse. By choosing easy plants there is nothing difficult in them. Some driftwood would make nice hideout. You could also take few khulies for example to the tank with betta. Ok, 10gal is bit small for them but then again, they aren't actually very active.
Betta will need a heater. Temperatures between 79-86F are good. In cooler water bettas tend to get sick easily and live shorted life generally.
I recommend that you search some information about olaces where bettas live in nature. That is of course if you really care for fishes and wan't to make natural habitat to it/them. Personally I think making aquarium piece of nature gives biggest satisfaction. It's nice to keep happy fishes.
And for all who keep bettas in smaller tank. Please don't get mad to me. I have right to my opinion, son't I. Just like you. Bigger tank is not worse than small, I presume. I think it ethical to offer best possible conditions to your fish. Peace
Here's my betta:
He lives in about 25gal with tetras, corys and bristlenose. Looks happy, makes bubblenest. Temp is bit low for it (~77F) thought and I'm feeling quilty because of it. I need to find better place to it soon.
thapsus