I'm trying to produce a 30-40minute documentary film right now and I've never made one.
Before I give you the rest of my advice, let me give you one piece of advice that you will probably ignore, but you shouldn't:
Forget it. The project you've chosen to do will be a massive failure. I'm sorry, but somebody needs to be blunt and honest with you. You simply cannot succesfully direct a 30-minute documentary your first time out. You have no idea how MASSIVE a workload you're giving yourself. You might be able to put a 30-minute piece together, but without the prior experience of knowing what works and what doesn't, it will be boring as shit, and most viewers will tune out after a couple minutes.
Nobody has ever succesfully done what you are trying to do. I challenge any reader of this thread to prove me wrong. Show me ONE instance of a first-time documentary-maker who produces and directs a compelling 30-minute movie their first time out.
I'd say 3-4 minutes is a more reasonable timeline for your first outting, and even that will be a challenge to you. The rest of my advice applies the same to a short piece or a long one.
You cannot over-plan. In fact, you should try to over-plan. Leave no stone left un-turned. Research the subject intensely. Before you even sit down for the first interview, you should already have a rough, but strong, idea of how the narrative is going to unfold. You should already know the focus of your story, and how you're going to tell it. Don't be afraid to frame questions in a manner that will likely produce the answers you're looking for. Yes, this is manipulative. So what? The entire process of making a documentary is manipulative.
I agree with all of the advice that's been given to you already. Except for one little tidbit from Ericksen, saying that you should avoid talking-heads. Well, he's right -- talking-heads are boring. But you still need them; that's just the most effective way to gather interviews. So you get lots of talking-heads, but you try not to make that your main aesthetic. You get TONS of footage that is relavent and pertaining to what your subjects are talking about. Pictures can work for this, too. You try your best to decide on these images in advance, brainstorm all the possible ideas for what imagery will help tell your story. Predictably, once you've collected the interviews, you'll see that there are many pick-up shots you didn't think of, but it definitely helps to try your best to anticipate.
You know -- you can say a lot in 4 minutes. You can deliver a powerful message that can call people to arms. Limiting yourself to a shorter timeline will force you to really focus on what's important, and to deliver the message in a consice, compelling manner. For the purpose you're intending (and considering your lack of experience), I think a 4-minute documentary will produce MUCH better results for what you're trying to accomplish.