How do I go about starting a business plan?

Every company needs a business plan and anyone who says otherwise is being lazy. The key is not to over invest in wasted stuff. If you don't have a plan you're not likely to succeed. No need for a lengthy Word document. If you can't answer the following in writing to yourself and your team then how to you expect to succeed?

• what problem are you trying to solve and for whom?
• what solution will you create to solve this problem?
• what economic value exists for the people who would use your product and how will you therefore make money?
• if you're successful how big can your market be?
• how much will you charge for your product and how does this compare to other alternatives?
• which customers will you target and how?
• who will you need on the team?
• how much will it cost you to acquire customers and when you get them what will be the lifetime value of those customers.
• etc.

You then need to turn this into a Powerpoint deck to raise money. I cover the outline of what you need in this deck on my entrepreneur & VC blog with advice on how to raise venture capital

Do you need a business plan? Maybe you don't!

If you can elaborate on what you hope to achieve by writing one -- getting funding, seeing if the idea is viable, etc -- perhaps we can suggest ways of achieving the real goal.

Others questions about what you're trying to accomplish are spot on. That will determine the end format and the scope of it. But whether the business plan needs to blow people's socks off or not, you have to know the answers to what should be in one even if it's all just in your head. Mark's answer has a good outline of many of the elements of one. If you can't answer those questions then you're hurting your chances for success. It has always surprised me how many companies don't have a business plan (and a marketing plan) - even just in their heads.