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.
A successful business is raised on the foundation of a great marketing plan. Selling is the most crucial and also the most difficult aspect of any business. Marketing of a product or service is real hard work and a good marketing plan is what ensures that the hard work bears fruit. It outlines how a business will go about attracting prospects, converting them into customers and retaining these customers in the long run.