What is the difference between a notary public and a commissioner for oaths?

In Alberta, a notary public can do whatever a commissioner for oaths can do, but a commissioner for oaths may not have all the powers of a notary public. In Alberta, a notary public is automatically a commissioner for oaths, but the reverse does not hold: a commissioner for oaths is not automatically a  notary public. A notary public can certify copies of documents and can act as a witness for documents that are to be used outside of Alberta. A commissioner for oaths generally cannot certify copies of documents and should not typically act as a witness for documents meant to be used outside of Alberta.

Do I have to appear in person before a notary public to get a document notarized?

Typically, yes. A notary public can be thought of as a professional witness. When a signature is to be witnessed, the person signing the document must appear before the notary public, and must prove his or her identity, usually with government issued picture ID such as a driver's licence or passport.
For documents such as certified copies, rather than requiring a person to appear before a notary public, an original document must be viewed by a notary public.

Will you notarize or act as a witness for a will prepared by me in advance?

Typically, no. A will is an important legal document that should be drafted with the assistance of a lawyer. While it is not a legal requirement to have a will prepared (or for that matter witnessed) by a lawyer, it is a document that has such significance that under almost all circumstances a lawyer should be involved with a will's preparation. Also, when a lawyer assists in any way with a will, that lawyer runs the risk of being sued by disgruntled beneficiaries at a later date. It may not be an adequate defence that the lawyer was only witnessing the will and did not assist with the drafting of the will. 
For those reasons, if I am to sign as a witness on a will, I must be satisfied that the contents of the will are reasonable. This falls within the realm of providing legal advice, and to do an adequate job, requires that some time be spent considering the draft will. Almost invariably, when reviewing a proposed will drafted by a client, I come across sections that should be improved upon, and it is in many cases more economically efficient to draft the will from scratch.