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.
Marketing Research will be the first component for your marketing plan. You may be doing this in house or you may elect to hire a top marketing agency, a marketing consultant, or maybe a specific marketing research firm. You need to obtain and capture all data about your business along with your products and services to help you properly facilitate any potential marketing problems. You may also be able to get some marketing data from an industry trade association or from other businesses within your specific industry.