[Ietf] SIP Stack start off

i am a fresh computer graduate. i want to develop an application using the free 
available sip stack in /c++, can anyone help me in deceiding a better one and 
provide me the link for the same.
i also want to have a proper documentation of the satck and plz plz tell me how 
to start off with it. What functionality and concepts do i have to make use of.