You can't tidy up RAM, Windows manages that.
You can tidy up disk storage space. The quickest thing you can do that will have a big impact is to copy everything in your C: drive data folders, i.e 'My Documents', 'My Pictures', 'My Music', 'My Videos', etc. onto your external HDD. Once you are sure and only once you are sure, that the copy is successful, go back to your C: drive and delete everything in your C: drive data folders and then empty the recycle bin.
1) You can reorganise the data files on your external HDD to your hearts content.
2) Only ever save stuff to your C: drive data folders as a temporary measure, make a habit of moving stuff to your external HDD as soon as possible.