You can just wash it with hot water and lots of scrubbing! But if you're really worried one part vinegar for every nine parts of water would work, and make it extra safe.
Depending on how big the tank is you may be able to cycle it, if you know your tank's dimension I suggest you type it into this calculator
to see how much water it'll hold. 5 gallons and bigger tanks will be able to cycle, if it's smaller than 5 gallons, don't worry about it.
I'm also going to point out about getting a heater, Bettas are tropical fish, and I'm pretty sure that you don't live in a tropical area where the weather is always in the high 80s. So be sure to pick up one, it'll not only heat the water but keep the temperature stable, fluctuations in temperature will kill fish. Along with a heater, get a thermometer to make sure the temperature is where it's supposed to be, those in-tank ones work the best. As for water regular tap water is fine, you'll need to pick up some dechlorinator, I suggest Prime, and Stress Coat.
That's all you need to know really.