Look over all of the courses that fit within the category you're trying to fulfill. For example, you might need to take one science course. This is a very broad category, and there will likely be dozens or even hundreds of courses from which to choose.
Eliminate classes that you have no interest in or that you are not qualified to take. Some classes will require prerequisites, for example, that you may not have. Once you narrow down your options, comparing the classes will be much easier.
Find out when the classes can be taken. This will help you compare classes and decide which ones don't fit into your schedule. For example, if a course is only available during your chemistry lab, which you need for your major, then you won't be able to take it.
Diversify your schedule. Compare the various categories that you can choose from and pick from among them. For example, if you are required to take three math courses, don't take them all during the same term. Instead, take one math, one humanities and one writing course so you have more variety in your schedule.
Talk with others to learn more about particular classes you're comparing. For example, you might be trying to decide between taking sociology or psychology to satisfy your humanities credit. People to talk with include students who have taken the courses, your academic adviser and the professors of courses that interest you.