In order to take backup of Lightroom catalog and related files.
> Navigate to the Catalog location
> Copy the entire folder and paste it on a external hard drive.
(Make sure that you have .lrcat file copied successfully)
Default Folder location for Lightroom related files.
Alternate method :
You can also backup your catalog on a external storage.
NOTE : You also need to copy the images to your storage device additionally as Lightroom doesn't include the image files.
Hope this helps.