This time I'm very optimistic about new major macOS update, I believe this version will fix for good the issues from High Sierra and bring back rock solid stability of macOS which we were used to from the past.
But there are still few things to consider before taking a step into Mojave...
Yes, that's important to keep your backups safe and up to date in general but it's essential before major OS update like this.
Time Machine is good option, which could be nice and easy configured and is available right in your macOS.
Have your current macOS version bootable installer ready.
It's incredibly good idea to keep installer for version of macOS, the one just before major update like we have now 10.13.6 of macOS High Sierra. Those are usually most complete and stable versions. Unfortunately some of those older versions of macOS are not that easy accessible unless you have them in your Purchased apps.
If you are still with macOS Sierra, which is in my opinion the most stable version with minimum issues these days, download Installer for macOS Sierra version 10.12.6, fortunately it is still available from the Mac App Store.
So either version you have, before upgrading to Mojave, go ahead and make your bootable installer for macOS ready.