If you haven't checked them lately, you may inadvertently install High Sierra before you have prepared properly. If your App Store settings have "Download newly available updates in the background" enabled, you will soon get the a Notification with only two choices: "Install" or "Details"
If you have DISabled the above pref, you should get this Notification that offers "Details" or "Not Now".
Note the difference in your choices!
With either Pref setting, I highly recommend you click "Details"! There are several things you should do before Installing a new Operating System, those things can be discussed below. Many have reported problems after installing High Sierra on the machine running WeatherCat.
The important thing is that by choosing "Details" is a not so intuitive way to avoid installing in the first case. Simply click on "Details" and your App Store will open. Now, Quit the App Store. Now, UNcheck "Download...in the background". Please.
The cause for this unethical, at worst, or unsafe, at best, behavior started with the update of Sierra. Apple is literally pushing us to upgrade, whether we want to, are ready to or not.