Everyone has their own way of learning, so you might want to start with taking practice tests and learning only those topics that you got incorrect answers on. These topics are indicated at the end of the practice tests.
Or you might start by taking lessons and then taking the practice tests. It’s up to you. Keep in mind, if you discover that three months is not enough time to prepare for the GED test, you can always renew your subscription and receive a 20% discount!