Familiarize yourself with terminology. The test is based around two general categories of personality, each with eight subsets. These two categories are extroverted and introverted. These terms, first introduced by Jung, refer to personalities which are either outwardly or internally oriented, respectively.
Study the types. The test is based on how an individual prioritizes or uses the "hierarchy of mental functions." These functions are thinking, feeling, sensing and intuition. The personality type will begin with either extroversion or introversion, then follow with two of the above-mentioned functions, one primary and one secondary. For example, a person may be "extroverted intuition with thinking" or "introverted thinking with intuition."
Examine dominant and supporting function. The secondary, or auxiliary function, assists the dominant function. This is because, according to the MBTI, the individual prefers to engage with the world through that primary function, yet calls forth the auxiliary in support. So even if the MBTI seems to reveal only a slight difference in personality type--like extroverted thinking with intuition and extroverted intuition with thinking-- those two people can in fact be quite different.
Take the test. There are a number of online sites that offer the test. One of the best ways to become familiar with this material is to actually go through the examination. However, you should be aware that this is not an official test. Only a licensed professional analyst can properly administer the test. Additionally, it is deemed unethical for a professional to administer an MBTI without a "person to person follow-up."