Ideally the tank should be at least a 2.5G tank (I personally prefer a 5G,but we could all argue all our lives about sizes
).The temp should be a constant temp (mine are 78 deg) a heater will do this.Also there should be some cover for the Betta so it can feel secure e.g. a cave or plants (If you use fake plants try to use silk ones,if you use plastic ones make sure the edges arent sharp enough to rip its fins).
Answers from the forum may be a bit sharp,but they are concerned about the welfare of the fish and the fact that people buy fish (living creatures) without researching about them first.On the reverse side I can see your perspective (its not yours,but you care enough to ask questions to help its welfare..so thanks for that,sorry about any flak its not personal).
Please follow the advice from the posts above,especially the water changes at least!!