You would first do a P&L for the present year for your existing business and the first year and as many years after as you would like to have your plan cover. Your existing business financials will be the foundation for building yourself a business plan for as many years out as you want.
The internet is a fantastic resource for writing a business plan, but for some people, nothing beats a good old-fashioned book. Your local library has entire sections dedicated to the multiple aspects of business development, and you can be sure to find several books about how to write a business plan. Best of all - these are free! If your local branch does not have the book you're looking for, check the catalog and request a book transfer. Sometimes, the perfect books about writing an effective business plan or ideas for a sample business plan are just an inter-library loan away.