One of the most reported macOS Mojave problems is a Mac getting stuck in the middle of the Mojave installation. A simple explanation is your Mac is just not technically fit to run Mojave. According to Apple, macOS 10.14 can be launched only on Mac not older than from 2012. If yours was released earlier — sorry, you’ll have to miss this update.
Fix 1 . Try reinstalling macOS Mojave from scratch.
Fix 2. You have downloaded the macOS Mojave and still cannot go past “Preparing the installation…” window. Let’s admit, new macOS versions are often buggy and problematic but here are a few more ideas to try.
Start your Mac in Recovery Mode. To enter Recovery Mode on Mojave, Click Apple logo > Restart. Right after that hold down Command + R buttons while your Mac is restarting. While in Recovery Mode you’ll be able to run Disk Utility to check your disk for consistency. Disable your antivirus software if you have any installed. Sometimes such apps would block any new installation attempts — a classic case of over protection.
More details here