The out from the SM58 is XLR - you should be able to find an XLR-1/4 inch cable, or you can get a standard XLR cable. From there, you'll need either a 1/4 inch to 1/8 inch adapter, or a XLR to 1/8 inch adapter, depending on which cable you get. They may refer to any of the connection-types by other names, but these terms should get you where you need to be. Of course, unless you get a really incompetent employee, they should understand you're question as phrased in your post here.
[EDIT] Alas, beaten to the punch