Dear new developer,
If in doubt, put it under version control.
Version control is a way for you to keep track of the core nuts and bolts of code, which are files on the filesystem. Version control lets you make mistakes, err experiments, and roll back time to when things worked. Version control lets you see who made what changes when to which files, so that when you need to find out who to ask about the bizarre function in class XYZ, you can.
Don’t be surprised if that person is you.
There are many systems of version control. What you use depends on your needs, the most important of which is how many people are interacting with your system. But something, even a crufty old system, is far better than nothing.
There are plenty of free version control systems out there, so if you aren’t using something, set up one of the free ones (or better yet, use a hosted service) and give it a try. Github, bitbucket, gitlab, all are happy to walk you through how to store your code in their system. Learning one of the modern version control systems will help you integrate into any team, because running a software project of any size without version control is a foolhardy endeavor that only is typically tried once.
There are some items that don’t belong in version control. Large binary files belong on a storage system like S3. Secret information, like a password, is better kept out of version control and in a secrets manager or environment variables. Documents that are going to be edited by nontechnical users should probably use a shared document repository (which will likely have versioning built in, but hidden).
But everything else, all the source code and build scripts, all the infrastructure setup and documentation, all the assets and sql scripts, those should be kept in version control.