Though it doesn't seem like it, writing a business plan for your type of business is, for the most part, similar to writing a business plan for any other business. The differences will mainly be that you may need to change the financials so that they fit a non-profit business. In that case you'll need a Funding Forecast instead of a Sales Forecast and you'll replace the Profit and Loss items with Surplus and Deficit.
I think you'll find lots of relevant information and help in the "How to write a business plan" section of our How to Articles. You can find that section here:
And like all business plans, use what is useful to your plan and leave out the sections or information that doesn't apply.
[Editorial note: We do try to avoid making sales pitches, but this question is exactly why Business Plan Pro was developed and is the market leader. Business Plan Pro has a toggle to switch from for profit to non-profit plans so it makes this easier.]
See if you have a local Small Business Development Center (SBDC), Business Resource Center or SCORE office to get advice on where to begin. You can check the list on Bplans.com (http://www.bplans.com/business_planning_resources/sbdc/ or http://www.bplans.com/business_planning_resources/score/). Your local Chamber of Commerce office should be able to provide some resources as well.
On Bplans.com there are over 500 sample business plans that you can learn from.