The subject matter of your documentary is the most important aspect of the film. You can choose a subject or issue of which you have little or no knowledge. This will help motivate your inquiry into the subject. You can also pick a subject about which you have strong feelings and examine a viewpoint that is in opposition to your own. You should research documentaries produced over the past 10 years to make sure the subject you choose hasn't been explored in a similar fashion.
Plan out the kinds of shots you will need to tell the most compelling story. Some filmmakers favor more close-ups to create a sense of intimacy with the audience. Others prefer to use wide, panoramic shots that evoke a sense of space and freedom, especially in nature documentaries. You should also decide the kind of film stock you'll be using and whether you'll be shooting in black and white or color. Most professional films use more expensive 35mm film stock as opposed to 16mm, which doesn't have the same sharpness and resolution. Newer digital formats can be more affordable, but they may not have the crisp look of 35mm.
Research the budgets of produced films that are comparable to the film you are planning, and obtain a copy of their actual budget breakdowns. Buy a film budget software program that can create a working budget by systematically guiding you through all the costs associated with a film. Jungle Software (junglesoftware.com) sells budgets for all kinds of films, including documentaries; its website offers a free sample budget for a $200,000 documentary.
With a budget in hand, you should begin securing the financing to make the film. You will need to make a proposal with a treatment that states the reason you believe your film is compelling, how you will shoot it and, most importantly, why you think it will be profitable for any potential investor. If your subject matter has great public interest or is a hot-button issue, you may be able to approach state and federally funded arts councils or other grant-making organizations. For example, in January 2011, the Ford Foundation announced the creation of JustFilms, a five-year, $50 million initiative to find and support filmmakers whose works tackle important social issues.