Allow me to give you some advice that will be different than what you may have heard and want to hear.
As I wrote you previously, and Nick has confirmed, you should have NO need for a soundman. Just make sure that your cameraperson/DP is always wearing headphones so they are constantly able to keep track of the quality of the sound being recorded.
That said, I don't understnad how – if your crew will be just yourself and the DP – how you think that you could be recording sound, especially during interviews. Who''l be asking the questions?
Even when you're out and about shooting B-roll, you should be watching what your DP is shooting and watching everything that's going on around you. I'm convinced that it would be a big mistake for you – if you're an absolute beginner, as you've described yourself – to worry about recording sound. There are many other far more useful things that you could be doing instead.
First of all, during interviews, concentrate on the process: make sure that you are listening carefully to your subjects, make sure that you're getting the answers you need, that you've asked all the right questions, etc.
When the DP is shooting B-roll, you should be keeping a checklist on shots, thinking if you've got everything you need, about what else you need, how the different elements (visuals, interviews, etc.) will work together, thinking about how to shoot a particular scene so it will fit in with your story etc.
Some key points here.
1) Preparation work and research can be the make-it-or-break-it factor for any documentary. Have you researched your subject well enough? Do you know what to look for, where to find it? That said, learn to be alert and open-minded because many times, in the field, a story can take an unexpected turn and you need to be ready to see it happen and follow the story down a different lane than the one you'd expected.
2) Do you have a story concept, do you know why you are shooting this documentary and what you want to show us? If you do, you certainly haven't told us! Do you know how to visualize that concept? Have you decided what visual elements you will need? Have you prepared a hypothetical shot list based on your concept? Remember the golden rule: "Show me don't tell me."
3) Treatment. Have you written a treatment? Barry Hampe in his MUST READ "Making Documentary films and videos" writes that a treatment "sets forth the idea of the documentary comprehensively enough to be understood , but with enough flexibility to allow for chance, change, and the occasional flash of creativity. A treatment is often referred to as an outline for a documentary. But it's much more than that. It's really an explanation of the documentary. It describes the content of the documentary and the style in which it will be shot. What it is about. What will be included. How it will be shot. And what il will look like. It includes all the elements – the people, places, thinsg and events – whihc must be a part of the documentary. And it tells how the documentary will be organized to communicate with the audience."
3) Communication. In addition everything else, communication between you and your DP is a crucial factor. Unless you have told her/him everything about the story, what you want to tell and how you intend to tell the story, s/he is shooting with a partial blindfold.
4) Ask for advice. Find someone who has already directed/produced several documentaries that you can trust and be open about what you want to do. Tell them what you would like to do and seek advice.