A great place to start is Home Page - IntelliJ IDEA Guide or for the full online help you can read IntelliJ IDEA overview | IntelliJ IDEA.
One of the most important things to learn in an IDE is how to debug the code you are writing, or more specifically, what debugging features does the IDE have. Fortunately IntelliJ IDEA has some great debugging features, which are very well explained in the following videos:
Building on that, it is worth looking at Troubleshoot Slow Debugger which even if you are not having performance issues, is still a very interesting read.
I recently upgraded from JDK 1.8.0_172 to 1.8.0_181 on my Mac via brew. This removed the previous version and so my IntelliJ projects lost the plot. To correct this I expanded "External Libraries", highlighted the JDK and picked "Open Library Settings" off the context menu, from here I could correct the Java home directory.
I have done some work with a Java project that used Project Lombok but IntelliJ did not cope very well with it. So I made two changes. First I went into the Compiler options and checked "Enable Annotation Processing", I then installed Lombok Plugin :: JetBrains Plugin Repository, which seemed to make the difference.