Ive written countless business plans and probably the best site ive used for business startup reference is the Government backed site for US Small Business Administration . They have a jam packed site for the small business and a section about writing a business plan.

Writing a business plan is essentially writing a diary for your business over the coming years. Your diary or planner should detail what you expect from your business and what your plans for the business are.

To help you out ive taken an extract from the US SBA - http://www.sba.gov/smallbusinessplanner/plan/writeabusinessplan/SERV_WRRITINGBUSPLAN.html

Elements of a Business Plan

1. Cover sheet
2. Statement of purpose
3. Table of contents

  I. The Business
               A. Description of business
               B. Marketing
               C. Competition
               D. Operating procedures
               E. Personnel
               F. Business insurance

II. Financial Data
               A. Loan applications
               B. Capital equipment and supply list
               C. Balance sheet
               D. Breakeven analysis
               E. Pro-forma income projections (profit & loss statements)
               F. Three-year summary
               G. Detail by month, first year
               H. Detail by quarters, second and third years
                I.  Assumptions upon which projections were based
               J. Pro-forma cash flow

       III. Supporting Documents
               A. Tax returns of principals for last three years Personal financial 
                   statement (all banks have these forms)
               B. For franchised businesses, a copy of franchise contract and all 
                   supporting documents provided by the franchisor
               C. Copy of proposed lease or purchase agreement for building space               
               D. Copy of licenses and other legal documents
               E. Copy of resumes of all principals
               F. Copies of letters of intent from suppliers, etc.