Once you find a minor that you might be interested in, you should speak with an adviser in that department. The advisor will let you know exactly what is required to satisfy your chosen minor.
An advisor will also give you information on what courses are required for the minor, and how the minor might relate to your major. This is important because you may be able to plan a few courses that meet both your minor and major requirements. You will also discover the prerequisites for some of the courses in your minor. Some minors even have GPA requirements that an advisor will be able to let you know about.