The best places to look are your Documents, Desktop, Downloads, and Applications folders. Some apps would store files in your Application Support folder, but you would never want to remove anything from there unless it is a folder for a specific app that you no longer use and plan to never use again. Don't guess. Those are found here:
~/Library/Application Support
These 3 folders may contain files that launch on startup and typically are not large files that waste storage space, but may affect performance. If you installed an app that requires a file in these locations, it may cause the app to no longer work correctly.
~/Library/LaunchAgents
/Library/LaunchAgents
/Library/LaunchDaemons
To access these folders, you go to the Finder, then the Go menu, followed by Go To Folder. Paste each of these locations there and don't forget the "~" character on some of those. You should make sure you have a backup in case you delete a file that is required for normal operation. Never use a Cleaner app that claims it will clean your computer. The worst of those is called CleanMyMac, which is known here as BrickMyMac.