I was able to get certified on Sitecore 9 last year (2018 on October), and this is an attempt to summarize what I did so you can hopefully have a better idea of what to expect.
First, the certification is called “Sitecore 9 Certified Platform Associate Developer”. I started by looking at the different web pages Sitecore has as preparation and turns out this page has everything you need. So, I am done here.
Just kidding (that was really bad, I know :)). But really, this is the ultimate written guide for the test. You definitely need to take a look, it contains the topics it involves, few example questions, and some awesome links per each topic.
This brings us to the basics of the exam. It consists of 50 questions and 90 minutes to answer them. The tricky part is this needs an external camera, and an account on Kryterion. But nothing to worry about, everything is on the above link.
In addition to carefully reading the link, I was able to take the official Sitecore e-learning course which is my next suggestion to you if you have the opportunity and money. This really can leave you in a great position to take the exam.
The official course obviously contains all the topics involved in the exam, plus some walk-through videos and labs. Everything you need. Now one more final advice, take it seriously. If you haven’t worked with Sitecore before it is possible you can still pass the exam if you follow the advice, but if you had worked with it. Then you are good.