voip/ calling card system

hi is there any one can tell me more about voip. if i want to provide calling cards to the customers. what i have to go through. i hav broadband on windows xp, i am going to buy voip gateway either 4 port or 2 port. after that do i need to make any account to make any call, and how. how calling card works do i need to make any account and how customer can make call through my gateway.
thank you