Becoming a session co-chair is a great way to get involved with conference programming that interests you as well as become acquainted with Divisions or Forums in your field of interest.
A co-chair’s responsibilities are primarily to support the chair of the session. Please make contact with the chair as soon as possible so that both chair and co-chair are on the same page. Regular communication with the chair is important in making sure everything develops smoothly and that the co-chair is actually able to contribute to the process.
All in all, it's not a terribly difficult job, but does require some time commitment. Also, this is not set in stone, and some of these items and dates will vary between division, topical, and conference. If there are questions, feel free to ask the chair, area chair, or YP programming chair.