Welcome to AA!
First thing you need to do is learn about cycling.
Here is an article about it and proper tank maintinence.
Guide to Starting a Freshwater AquariumAquarium Advice
Keeping female betta sororities can be tricky. The best advise I can give you is to get young fish and to add them in groups. It's aslo best to have sisters from the same spawn as it cuts down on aggression issues. Provide them with plenty of hiding spots and tall plants to break the line of sight. 6+ females is recommended so aggression is spread around. In a 20g IMO
6-8 females with a group of bottom dwellers such as kuhli loaches or cory catfish would be ideal.