Start with articulating the overall goal and specific objectives of your proposed project. You should have one overall goal and 2-4 objectives. Then, back up the significance of your project with a review of the literature and the gaps that exist. As a start, write one paragraph. Also, in another paragraph explain how your project is innovative. After you have these three core pieces, look for a specific sponsor (of you're talking about a grant proposal) or turn to guidelines for research proposal writing.
Below are a link to proposal examples and a reference of an excellent book on academic/scientific writing.