Backup Your Mac
Back up your Mac. If you normally do this with Time Machine, make sure you have a current backup before you proceed. If you don't typically do this, you should take time to back your Mac up before you install the beta.
It's a good idea to go a step further and keep this backup separate so that Time Machine doesn't overwrite it. A good backup on macOS Mojave is the easiest way to leave the beta and go back to what is working.
Make sure your photos, documents and other data is safe before you install the beta on your Mac. Even if you backup key files and photos to the cloud, a local Time Machine backup is important.
Go to Settings -> Time Machine and make sure you have a recent backup. This is especially important if you use a laptop that you don't always plug in to a hard drive. Here's more on how to use a Time Machine to backup your Mac.