Hi there and welcome to the forum, and the wonderful world of bettas!!!! So you have never owned bettas before? If not the two most important things a betta needs to survive is clean water and a heater.
Bettas are tropical fish and cannot tolerate fluctuations in water temperature as this stresses them and compromises their immune system putting them at risk of all sorts of nasty and often fatal illnesses and diseases. The only way to provide them with the stable temperature that they critically need is by using an aquarium heater.
Water changes are very important too, bettas are beautiful fish but those lovely big fins come at a bit of a cost, they make the fish prone to bacterial infections if their water is not changed enough. A single betta ideally should have at least a 2.5 gallon tank which would need to be cleaned once or twice a week. A tank smaller than this requires much more maintenance and may need three changes or more per week depending on the volume of water that it holds.
Bettas need to be fed a nutritionally balance diet and the only way to do this is by feeding a pellet feed. Some brands are much better than others so just keep that in ind when choosing....cheaper is not always better. Freeze dried foods should be avoided as they are known for causing digestive troubles such as constipation (which can be fatal) and bloat. Frozen foods are a good way to add some added nutrition and interest to your fishes diet.
If you would like to know anything more please just ask and we would love to see some photos of your bettas if you could get some