My mom was a nerd. Some of my earliest memories are of watching the Star Wars trilogy with her. I will always associate those movies with love and warmth, and as a kid I watched them so many times my dad made a rule that I couldn't watch them while he was home.
Another part was the school library. In sixth grade, I found out they stocked Dragon Magazine, but kept it hidden under the librarian's desk for no reason I ever figured out. That they hid it made me all the more voracious for it, so I read it intensely.
In high school, I noticed some boys reading Wizard Magazine. The nearly full-page picture of Adam Hughes-drawn Vampirella they were looking at exposed me to feelings I wasn't ready to deal with, so I asked one of the boys I kind of knew if I could borrow the magazine to read it. He said yes, and I discovered I really like Superman. Things progressed from there.
There are other steps scattered throughout the middle; discovering Margaret Weis in junior high, for example, or my mom buying me my first ever hardcover books in the form of Timothy Zahn's Thrawn trilogy. I think she also gave me my first Stephen King book to read when I was ten, the Eyes of the Dragon. But, in general, I became a nerd because I kept noticing nerdy stuff and decided to fixate on it.