The best way really is to just learn things yourself while you go at it.
I very much remember my first days on HL2RP. I had no idea what the purpose was and I had no idea what serious RP actually meant. I just joined the server, and started to explore and ask people around for help to get started.
At first it seems really boring. Since all you THINK you are doing is just walking around and talking to other people. But as soon as you get to the point of understanding how to develop your character through serious RP, it starts to be really fun.