You need a tank of at least 5 US gallons. A 10-gallon tank is better.
a filter
an adjustable heater
a water conditioner
All of these are mandatory if you want to keep a betta fish.
Plants will not be enough to take care of the ammonia produced by even a single betta fish unless you cover every square inch of a 30+ gallon tank with heavy vegetation. The filter works for the aquarium nitrogen cycle, there is no other way around it.
Betta is tropical fish, and their preferred temperature is 78F+. They need stable temperature, and only a heater can provide it unless you are 100% sure your room temperature is always 78F+ 365 days a year.
For beginner fish keeping guides, check these articles out.
