Your original material is copyrighted as soon as it is fixed in a tangible medium of expression. There is no need to register that copyright with the Copyright Office in order to secure copyright. Registration does, however, give you some additional options. One can, for example, bring legal action against a web site that does not respond to your takedown request as well as against the students who may have posted your material. Information on registering your copyright and the fees to do so is found on the Copyright Office web site.