You have to first sign up for a free account and create your profile. Visit this official web page:
Once you have signed up, you need to complete the registration process. You will then be able to select your preferred test date and test center.
ETS allows you to retake the test as many times as you want. However, there must be at least 11 days gap between re-tests.
With the way that the tests are scheduled, the 11-day gap requirement means that you cannot take the test on two consecutive dates. Therefore, in one year, you can take the test about 25 times.