Version control is a must. Version control is software that keeps track of the files in your solution and tracks changes you make to them. It then allows you to save files while maintaining older versions of each file. It’s a true life saver if you accidentally make a breaking change to your solution - you can just roll back to a previous version. It’s also a great safety net if you are looking at doing some big changes to your code but don’t know what those impacts could be.
The most common source control these days is git. Git was created by Linus Torvalds, who also created the Linux kernel! Smart guy. Its adoption was certainly helped by GitHub, which is where I store all my public code. GitHub is a hosted version of git. Other source control systems include Microsoft TFS, Subversion, Mercurial and CVS.
Use the link at the top to learn git online.