"Failing to Plan is Planning to Fail" - Ben FranklinIf you have a business or want to start one, you should have a business plan.
It needs to be updated a lot in the beginning and not so much as your business matures.
You don't want to get analysis paralysis and not take action because your business plan isn't done. If you won't take action on your plan, then don't waste your time writing it. Use the Finish Then Add Details (FTAD) method to "finish" it ASAP.
Your business plan isn't a static document. If you're not a planner and won't follow it anyway then you may want to use an Agile Method.
Your best bet is to keep your costs down as long as possible. Keep your day job until you've shown the potential to get customers.
Test your ideas with small amounts of money and cheapest or free marketing venues.
Your business plan doesn't have to be elaborate or hundreds of pages. It just has to outline: