Navigate to
C:\Program Files\Epic Games\UE_4.18\Engine\Binaries\Win64\UE4Editor.exe
Run this file and it should come up, if you don’t see any of the above you didn’t install the whole engine yet. Otherwise If you don’t know how to navigate there I would work on your basic troubleshooting with computers since this program can get technical very fast. Don’t get discouraged, just be aware you need to be at least somewhat tech savvy.