Hi there!
I'm not sure if you mean designing a business plan or designing the branding and look of a business and or business website.
First, you definitely need to have a solid business plan that includes what your company is all about, your mission, and what exactly you do.
Once you have your services or products outlined, you will need to calculate pricing to figure out where your break even points are to start making profit. Once your business plan and pricing is set, you can start designing the business marketing materials themselves, such as cards, website, ad campaigns, etc...
Start with the basics for your business branding and marketing. Choose a handful of main colors, develop a logo, or get one designed by a specialist. You can find great people on sites like Fiverr for logo design services and similar.
To run a small business website, I'd highly recommend WordPress. It's a free open source website management system that runs a lot of websites on the internet that you probably visit all the time. WordPress is also user friendly for non-coders. You can add and modify new pages fairly easily without code.
Make sure you build out your website with a solid homepage, about, and some service pages that explain what your business provides. Also make sure to include your main keywords that people use to search for your business in your website titles, images, and other web copy.
Lastly, when designing the website, make sure to think about what specifically you want the user to do once they get there. Phone call, form submission, etc... are likely options. Try to include elements in your pages that push users to your desired action of calling the business or submitting a form online, or what ever your goal is.
