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.
The marketing planning (concretized in the marketing plan) is an essential organizational activity, considering the hostile and complex competitive business environment. Our ability and skills to perform profitable sales are affected by hundreds of internal and external factors that interact in a difficult way to evaluate. A marketing manager must understand and build an image upon these variables and their interactions, and must take rational decisions.