[Frooxius]

Well then look at the link I mentioned. But what you want covers a whole range of various topics and various kinds of network communication. You can fetch some pages over HTTP, you can communicate securely over SSL, you can transfer files over FTP, you can operate with databases and send SQL queries to them (so you have to learn how databases work as well), you can be streaming some data, sharing a data and creating a P2P network, syncing some variables in real time when making a networked game etc etc... Each of these requiring a different additional set of knowledge.
The link I sent provides some basis for starters, but then you don't have just one beginning to learn from, but several of them, so unless you decide with which one of them you want to start with, I can't give you more specific direction.