The only way to get into Mensa is to document the fact that your IQ is higher than that of 98% of the general population (you are in the top 2%). You can do this either by providing Mensa with a valid copy of your score on an accepted standard IQ test or by taking the Mensa Admission Test, which is widely offered periodically by Mensa. Mensa is an international organization. The American Mensa website address is: https://www.us.mensa.org/
You need to have an IQ of at least 130 to have a shot at gaining entry. If you think you might want to take the Mensa Admission Test, I recommend taking the home test first. It costs $18 and is available through Mensa. The home test will give you an idea of how you might fare on the actual test, which costs $40 and requires two hours of your time (more if you must travel to the test site). The home test has fewer questions on it than the actual test, so if you score a little less than necessary to get into Mensa according to the home test, it is still worth your while to take the actual test. The Mensa Admission Test consists of two tests containing questions that require visual perception, logic, and deductive reasoning. If you score at or above the 98th percentile on either of these tests, you'll qualify to join. Mensa does not attempt to measure your IQ based on the test. This means that they will not tell you your IQ when you receive the test results. A qualifying score simply tells you that you tested at, or better than, 98% of the general population and that you are eligible to join.
You can also submit proof of your IQ from a test you took elsewhere. The following is taken from the Mensa website: "Results from tests given by an institution, agency or clinic must include the full name of the test, the score and the percentile rank. This documentation must be on the letterhead of the institution, agency or clinic; it also must be signed by the psychologist responsible for the testing and must include the psychologist's license/certification number as issued by the state in which the psychologist practices." There is a long list of tests that are acceptable for admittance into Mensa. Here is the link to the list at the Mensa website: https://www.us.mensa.org/join/testscores/qualifying-test-scores/. The list includes the Stanford Binet, ACT, GRE, GMAT, SAT, many tests administered by the military, and more. Mensa charges $40 to assess the test results and determine if they are acceptable for membership.
Annual membership dues is currently $59. Honestly, there are not many benefits to being in Mensa. Membership will make you eligible for discounts from certain vendors and you will receive the Mensa Bulletin (an interesting read, which you can subscribe to without being a member); but the biggest benefit of membership is attending Mensa events where you will meet other Mensans. The people in Mensa are frequently quirky, always smart, and usually a lot of fun. In my experience, it's more a social club than anything else. There are local chapters all over the world and each chapter offers different opportunities to attend events and meet other Mensans.