Male bettas have been generalized for having long fins. While this is common in petstore bettas, wild male bettas are short finned. They need the shorter fins in order swim in the wild current. Long fin male bettas often have problems swimming due to their long fins picking up drag. So basing sex on finnage isnt always accurate. Try looking at the front of the betta. Under the head area, where the gills meet, males will have an extra skin that forms under the fish. Also by looking at the head, when you become more advance you can see the head and the males will generally have bolder in a way.
Good way to sex females is to look under them. Inbetween the two ventricles is a white dot or the egg tube. This is probably the best way to find out the sex of your betta. When you become more advanced, you will begin to notice that again by looking at the head of the fish, you can tell if it is a male or female. Females tend to have a smoother sytle head. On my shipment of females, i did receive about 30-40short tail males, so you cant always guess that if you order females, that it will be correct. Imagine trying to breed the short tail male with the breeder male. So it is important infinding out the sex of the betta.