Starting a project

I’m starting a project in school where I need to come up with a plan for a business. My business is a business that fixes and updates computers at home and for small businesses. I need to decide what kind of computers to buy, how many to buy, servers I need and networking equipment, along with a security and backup and disaster plan. I’m not sure where to start. I don’t know how I’m supposed to decide what computers to buy and how many servers I’ll need. Another more specific question is in a small business can you have one server that holds all data for the different departments ie. accounting, sales etc? Any help would be greatly appreciated.