If you know you need soundproofing but dont know exactly what your problems are or why they are occuring, or if you want to know how to solve problems and what materials are available. Here is the place to start.
General information on sound, vibration, acoustics, and soundproofing
Soundproofing / Noise and the Law
Soundproofing / Noise and the Law sub index
General soundproofing guides for common problems
Soundproofing Tested fitting systems