Follow instructions of @Ronasara.
If one can not get Ethernet and Have to use Wifi - iffy but doable. Shutdown computer. Shutdown Router. Disconnect ALL Other devices that use the Wifi. Restart Router. Move laptop as close to Router as possible. Restart laptop and Hold the - Shift - immediately. This is Safe Mode, does a Repair Disk, clears cache files and only loads Apple Software and Fonts. It will load slowly - normal. Once logged in it will look and perform differently - also normal. Open the Apple Apps Store and download the Full Installer.