You need to know everything obtainable about your customers. This list will get you started. You will need their age, sex, marital status, income levels, education levels, geographic locations, where they shop, if they go to church, if they have kids, ages of children, political affiliation, what they like about your company, what they don't like, why they shop with you, what they value about your products/services, what they like the most/least about products/services, and whether prospects are unaware of your brand or products/services. Customers want you to know certain things about them and if you do, they will remain loyal to you as you take time to know them.
What was the outcome? As much as 90% businesses started by entrepreneurs and small business went busted or failed within a year. I experienced one of the many busts few years ago (with my cafe business) because i don't really have a proper and "the few solid sheets" called a marketing plan.