First of all you need to decide that what is the basic concept of your documentary, what is the angle that you want to show?
For example -
You want to showcase the hip-hop scene in Seattle but what is your documentary all about?
Is it about the young talent that is trying to make it big and their lives (how they go about it)
or, is it about the financial opportunities in this field?
or, is it an overview of the hi[p-hop scene in Seattle? etc.
Your documentary may have one or multiple things to cover but it is better if you keep it clear in your mind and in your film that what is the angle that you want to hold onto.
What you need to do is first of all jot down as specific points that you want to convey/ highlight through your documentary.
Ex - The points may be anything such as: Daily lives of the young talent or, Opportunities in this field, etc.
There are two ways of scripting it:
Either write your script first and make it very clear that what kind of visuals you would require to support your script.
or, decide that what points you want to cover and then just go out and shoot, get as many visuals axs you can and later on write your script according to the visuals you've captured.
Both the methods are good and it is upto you to decide what is better for you and the topic that you've chosen.
I hope my suggestions are of help to you. Please send me your email id so that we can talk about your documentary and also some of my projects.