Some friends and I are making a game and we would like to add a online multiplayer component, the problem is we don't know where to start finding the information, does anyone know where I can find a good tutorial, and/or book that will teach me the language and/or API(s) that I have to konw? Furthermore, I have been directed to TCP/IP and Berkeley sockets by different sources as good languages to learn so which would be better, and where can I find a book or tutorial for this language, that is preferably geared towards game programming?

Which programming language are you using? Java, C# and Delphi provide pretty good networking libraries, so if you're using one of those just look through the documentation to get started. I'm sure there are plenty of C++ libraries that do it too, if you have one of those it ought to have tutorials.

TCP is a protocol, not a language. You'll want to use either TCP or UDP for your game. Berkeley sockets are a way of providing an interface to your program; I think all 'normal language' networking libraries use the Berkeley model, but I might be wrong there.

If you want to save yourself a lot of time and headaches, I would look at RakNet. I have gone down the tcp and udp paths and, while learning about it is worthwhile, you might find using an established library to be much more favorable in the long run. I and some groupmates are usng RakNet for a school project and have found it to be of great help.

If you want to start out there however, google for beej's networking tutorial. It is a good start.