You Can download the Launcher Here
When starting the launcher for the first time you’ll need to tell the launcher where your FiveM folder is, as well your GTA5 folder.
FiveM will install at C:\Users\yourname\AppData\Local\FiveM
GTA5 will install at C:\Steam\SteamLibrary\steamapps\common\Grand Theft Auto V
This will be the case unless you have done a custom install.