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.
Let us see what do we call a "marketing plan"? It is the result of the planning activity, a document that includes a review of the organization's place in the market, an analysis of the STEP factors as well as a SWOT analysis. A complete plan would also formulate some presumptions on why we think the past marketing strategy was successful or not. The next phase shall present the objectives we set, together with the strategies to achieve these objectives. In a logical sequence, we will further need to evaluate the results and formulate alternative plans of action. A plan would consist in details of responsibilities, costs, sales prognosis and budgeting issues.