Make Sure You Know Your Passwords
When you finish installing macOS Mojave you will likely need to enter your Apple ID password to complete the process and log into all the cloud services that are a part of macOS.
Make sure you have your Apple ID and password handy, and that they work, so that you don't run into issues during this part of the process.
You can login to iCloud.com with your Apple ID and password right now to check and see if it is working.
It's also a good idea to have other passwords handy as you may need to log into apps and websites again. If you use a password tool to save them, you should be safe, but it's a good idea to make sure you have them available on your phone too.