VatSim And Avc

i need help using vatsim and avc. i dont understand alot about it, can i only use it when there is a "fly-in" or can i use it when i want and if there is some ppl on the same server, where can i find the VATSIM Authentication servers that avc askes for and what is a avc voice server and where do i find them and how do i connect to a server to fly online using squarkbox etc. help is needed thanks