A business plan is really a road map of what you want to achieve and how to get there.
What to consider when writing a plan
How your plan looks, whether it is written or just a series of post-it notes stuck on your fridge, is not vital. What is important, is that you know what you need to do and how you are going to achieve it. More importantly, it needs to be something you review and amend regularly, as your business develops. It should not be relegated to a drawer once you have completed it.
If you want to find finance for your business, you may well need to present your plan to a funder (such as a bank). This should show that your business has a solid foundation. This plan will need to have financial forecasts (.xls | 257kb) that are comprehensive and based on realistic levels of achievement.
See this example of a start up business plan (.docx | 42kb), to give you an idea of what you may need think about.