Think about it... you would normally have an exercise book for each subject. This makes sense, because you can keep your mathematics notes separate to your geography ones, for instance. You need to apply the same principle to the way to organise your digital work. Whether you're using Google Drive, Microsoft OneDrive or something else, make sure you have a folder for each subject and always put your work, notes, 'handouts' - in fact everything you want to keep for that subject - in the correct folder.