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.
Most of your marketing research will fit into 5 or 6 categories: Surveys, Focus Groups, Interviews, Observations, Secondary Research, or Experiments/Field Trials. We will not go into specifics for each research category on this particular blog. Regardless of the marketing research methods that are used, you need to uncover who your customers are or will be, you need to uncover what the demand is for your products or services, you need to know every competitor in the marketplace, and you need to know the market share for each company in your industry categories.