A business plan is an excellent starting place to identify your greenhouse's goals and begin your designs. Write out your goals, vision, and purpose. Identify your market and analyze it's strengths, weaknesses, and potential growth. Discuss how you intend to source your funds, and how you'll use what you are able to have invested.
A business plan should also clealry define your products and services. Decide who your primary audience and market will be, and how your products will fit into their life and needs. Develop a sales and advertising strategy, how will you define your business and make it known to the world?