Download skype, then get a friend to download it. Then exchange account usernames and add eachother... and make a call to the other party. Congrats, your using voip.
I think you may be asking more along the lines of hacking voip... you'll have to be more specific.
Also for future notice, VOIP = Voice Over Internet Protocol and the best place for information on voip and associated protocols, especially with a view to exploiting them is in the RFC's published by IETF.
@ReverseEngineering , please do not keep asking for someone to teach you such broad topics. Perhaps a better way to phrase it would be "Does anyone know good resources for me to learn voip?".