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.
Guide _ in a marketing you will think, record and document all your ideas, products and services offering, price, distribution channel, programs, sales direction, customer segment and so on. I have experienced whereby I suddenly realized that my business is not going on the right direction because I didn't follow my marketing plan. Have you experienced that? Or perhaps you suddenly realized that the thing you do know supposed to be done many months ago because you forgot about it? And you forgot because you don't jot it down and plan for it? This is what I mean that marketing plan as a guide. I cover this in my Marketing Plan Breakthrough S6PEC.