A solid business plan is a must-have and an invaluable aid to getting your thoughts in order. This will be an ongoing part of your planning and will need many revisions. Make friends with it and don't worry about making it perfect yet. Some key elements:
1. Mission Statement: Why do you want to start the business?
2. Company Goals: What are your critical milestones for success?
3. Business Type: Sole Proprietor, LLC, corporation, non-profit, partnership?
4. Financial Details: Startup costs, Profit & Loss, financing needs, …
5. Marketing Plan: Who is your target customer and how will you reach them?
The less you know about your industry, the more critical the business plan is. Though it is always important.