Git
One of the most useful tools for any modern developer is version control. It will save your life just as often as it will give you a heart attack if you don't know what you're doing.
The most common by far is a software known as Git. How does it work? How should you use it? What even is version Control? Well take a look at this note collection for a bit of help with those questions!
I wrote these notes while learning so they might be inaccurate or outdated. If so, let me know, and I'll try to fix them up ASAP. Feel free to make an issue or PR in the site repository to help me out!