Inventory management in Rails
Try to put 'on paper' what you 'fields' that you need. Once you have that then you must group them based on what fields have a same identifier. In you case you might have 3 entities:


User with an ID and a name
Group with an ID and a name (with values like like pen and paper)
Material with an ID, the GroupID and a name


For the ToDo you can try yourself to figure it out.

Google on database normalisation to find some more information. You do not need to go down all the way but try to figure out the first 3 forms.