"Otaku" isn't really something you try to become, it's more of a murky swamp that you just sort of gradually sink into if you have the right type of personality. Before you know it you realize that you're in so deep you might as well get comfortable. Anyway I think I roughly understand what you're trying to ask so here is my personal experience:
Around 2003 or so I kind of got hooked watching the anime that Cartoon Network would run at night (Inuyasha, Cowboy Bebop, Trigun, etc), and got interested enough in a couple of shows to start collecting them on DVD. When I started college the following year I saw a flier for a school anime club so I started going to that. Most of the people there knew a lot more about anime than I did and had been into it longer so I felt kind of out of place. However I got to be friends with some of the people in the club and I just kind of got sucked further and further into that world. In addition to the club we started going to conventions and the like, and we would frequently hang out and watch each others DVDs. I learned about 4chan from one of those guys too, and that was really the point of no return for me. Before I got into this stuff I was basically just a somewhat awkward and socially beta but still relatively normal male; I now live in a world of plastic figurines and erotic visual novels. My only close friends are the anime club friends I met in college.
Anyway, being otaku is basically like having any other interest that you're very deeply into, like being into a music genre or something. If you want to get into it just start looking into titles that sound interesting to you, learn all you can again, and go from there. One of two things will happen: either you'll find out it was just a phase, or you'll become one of us.