Decide If You're Upgrading or Doing a Clean macOS Mojave Install
There are two ways you can install macOS Mojave on your Mac. The first, which many users will choose is to do an upgrade. This installs the update and at the end all of your apps and data are still on the Mac. You can also choose to do a clean macOS Mojave install and give your Mac a fresh start.
If your Mac is relatively new, or you did a clean install for macOS High Sierra, you are probably OK to go ahead and do an in place upgrade.
For users who haven't done a clean install in years, or who have a lot of macOS High Sierra problems, a clean install may be the best bet.
If you go this route you will have to set up your Mac from scratch, but it can help you avoid bringing a number of problems with you as you upgrade. It takes longer, but it can be worth it.