I agree with Meatpuppet, even if your betta has survived a couple of months that could just be because bettas are hardy fish. I would go out ASAP and get a tank of at least 2.5 gallons, a heater, a filter, and a good water test kit.
If you keep up with the partial water changes (remember to use water conditioner or the chlorine in tap or bottled water will poison your fish) a 2.5 gallon tank will do fine and they are not very expensive. In fact you can buy a 3 gallon set up from a variety of companies that includes everything but the heater!
Don't feel bad that you didn't know all this before you came here, neither did I...pet store workers usually have no clue and that's why so many bettas live only a few months or a year.