Firstly I'd recommend that you look at the sample plans that are on this site. There is a boutique shop sample business plan already here which will give you good ideas on the next steps:
Secondly, 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.
As far as beginning, the success of any business plan is within the quality of the information provided. Once you determine the type of business you want to go into the plan itself is to represent the operation on paper as if it already exists. How much revenue is expected, from where, the marketing plan that prompts the sales, the infrastructure needed to meet the needs of doing business as well as other related actions and costs. Take your time and talk to as many people as possible for information. Please keep in mind doing your homework up front is like taking on an insurance policy. It increases your chances of succeeding by eliminating the unknowns. In the end there are always other variables.