recommend you read the various discussions we've held in this Forum.
The Al Maysles/Susan Froemke discussion would be particularly
valuable, I think.
I usually find it best to leap in by doing interviews first. They
don't cost much and preparing for them is a good way to bone up on the
subject at hand. I usually approach interviews by coming from a point
of innocence or downright ignorance, even if I'm familiar with the
subject. So it's not like you have to be an expert before proceeding.