My sorority had a crest drawn by a sister; and then just this year we got a professional design which changed the look considerably. I would suggest researching heraldry and the crests/coats-of-arms of other Greek organizations. From there you can begin to brainstorm...starting with the elements you would like to see in the design. From there, you can compile several sketches then have the group decide on a final design.
If you are looking for a more traditional look...you can go with a professional heraldic artist to create your crest. This is the route my organization took. There are companies like
http://www.fleurdelis.com/, or heraldic associations like
http://usheraldicregistry.com/, or individual designers like
http://www.dksdesigns.net/ (who did our crest
).
Good luck getting your crest created.