Read the blogpost I published in the Salesforce Developers blog.
In the same way I have elaborated an index of trailhead modules to work on if you want to get App Builder certification, I have created a similar index for Platform Developer I. Here you have the index:
Somebody asked me which trailhead modules could help for obtaining platform app builder certification. What I have done is to elaborate an index of them, in order you can practice some of the concepts that are asked in the certification. Obviously, doing this modules is not enough for passing the exam. However it is a good start point. Here you have my index: