I think from the moment you get interested by one song for example.. then you buy an album, another, and from the MOMENT you decide to do some research on the man, not just on his music, then you're.. caught in the fandom and it just gets bigger.
I always tought that to be a Michael Jackson fan, you have to understand suffering, criticizing and injustice, you probably felt it once, you care for others, you want to save the world and you want to hold this 50 years old little kid in your arm and tell him you love him...
When you have that, I think you have everything to be a MJ fan, and you probably will if you take that step into the 'out of ignorance' process.