A business plan is a written outline evaluating the economic viability of a business venture including a description and analysis of the viability of the specified business. Creating a business plan is an essential step for any prudent business person to take, regardless of the size of the business, or whether the business will operate in the "real world" or online. Creating a business plan is a process that is too often skipped but it is a critical part of your ongoing business progress. A business plan is not a guarantee of success but it is a fact that the vast majority of businesses that fail do not have a serious business plan.
Business plans vary enormously in scope and content but tend to follow a general format regardless of their size or complexity, or the size and complexity of the business to which they refer. There are numerous books devoted to business plan formats available in bookstores and libraries. It is worth taking the time to read at least a couple of them.
Most people with a start-up business seem to be reluctant to write down their business plan. You should find a good guide to creating an effective business plan -- there are several excellent suggestions on this site -- and get yours started right away. Your business plan will become the roadmap that charts the course of your business.
Of course you won't be able to predict all of changing conditions that will affect your business so, after you have begun your business and operated for a while, it is important that you periodically review and update your business plan. Don't let it sit gathering dust. Get it out. Go over it. Make changes and additions. Re-evaluate your original thinking in light of what is actually happening in day-to-day business. A business plan is a living, breathing, thing. You need to feed and exercise it regularly.