Betta splendeds are what we keep, AKA Siamese fighting fish. But there is a bigger genus of Betta which has other species, such as Betta Imbellis.
Your betta will be happier in a bigger tank! Like others have mentioned, they can survive in a small tank, but will thrive in a bigger one.
DO NOT put your tank in direct sunlight! For now, try to place his/her tank in the warmest area of the house, or use a lamp. When you upgrade the tank, it will be easier to find a heater.
Most petstore people don`t know what they`re talking about, btw. Just come to us when you have questions.
Sexing can be tricky sometimes. Females generally have shorter fins, and can get horizontal stress stripes (which are less common with males). One of the surest signs is and egg spot. Here's the best picture I could find of one of my girls where the eggspot is visible: