Uninstall games you don't need and remove by hand any game data that may have been left behind in the library folder. You shouldn't have this much space taken up unless you have a lot of games installed.

You may need to do a spot of drive cleaning as it rather depends what you've been doing. One common way of getting the problem where you have folders at silly sizes is if you've had games and downloaded loads of mods in the past. Because they're not officially part of any game, it's easy to get legacy data and errors like this.

There's many drive cleaning programs you can use so Google for what works for you.