Read the following articles for some useful tips to keep in mind as you are working on your documentary. Notice that in the first link, she says pick something about which you are passionate. That is excellent advice and will result in a much stronger piece.
In this second link, step three notes the order in which you edit your material is important. Think about what makes sense to you and what makes sense to the reader.
One more tip to keep in mind that is alluded to in the last link, keep your audience in mind, both when you are filming and editing.
Finally, remember to have fun!