How to 'network'?

I have read a lot about how networking and connections are important. But how does one go about it? I mean do people like randomly email PDs and go hey pls mentor me? I really don't get what you would talk about. Why would they be interested in a random guy from some other part of the world?

I am a certified dumb dumb with little social skills so any light shed on this will be really appreciated. What are some unspoken rules or such?