Create categories (essentially just folders) called "Archive" or "2013" or "4th Grade" or whatever seems appropriate. In your list of lesson plans, click and drag one title on top of another title, and the two will be combined into a folder called "New Category." You can rename that however you like, and then drag more lesson plans into that category.
You can also do this for courses and resources. All of those previously-created items are essentially just put away in a folder in a filing cabinet (metaphorically speaking), to keep things consolidated and tidy. You can create other categories too, for subjects, years, child's name, etc.