Google offers a set of free online courses in it’s Analytics Academy to improve your Google Analytics skills and to prepare for the GAIQ-Test. Here you will find video lessons from Google’s digital analytics experts and you can also test your own knowledge by answering questions and completing short quizzes at the end of each chapter. Keep in mind: a lot of questions (and answers) you will find here are also part of the GAIQ-Test!
So the best way to prepare for your test is to take these courses and learn about Google Analytics step by step at your own pace.
The courses are held by Justin Cutroni, Analytics Evangelist at Google. There are also some pre-recorded Google Hangouts with other noted Analytics and Digital Marketing experts like Paul Muret and Avinash Kaushik. The new Google Tag Manager course is held by Krista Seiden, Analytics Advocate for Google.