My 18 year old son was arrested for shoplifting at the mall, do we need an attorney? 

Anytime criminal charges are involved it is always advisable to have legal counsel. There is always a possibility of having the charge dismissed on a technicality or at least reduced.

Also, since he is a first time-offender he will probably qualify for a "diversion" or your state's equivalent.  Through this procedure he can plead guilty to the charge and then serve probation.  If successfully completed, his case will be dismissed and his plea thrown out.  This will leave him with either no criminal record or at least the ability to later expunge (ie clear) his record.

Right now I would speak to an attorney in the immediate area where all of this occurred.  They can best use their local contacts to your son's fullest benefit and explain tall of the above more fully to you.