There are many possible causes for armpit lumps.  You should probably start with your family practice doctor, especially if you already have a relationship with one.  It is usually easier to get in to see a specialist with a referral from a doctor, and your doctor probably knows the best surgeons and breast specialists in your area.  Your primary practice doctor can order an ultrasound and/or a mammogram, and it may turn out that the lump is a harmless cyst or enlarged lymph node that doesn't need any other treatment.  Probably your doctor will send you to a general surgeon for a biopsy of the lump.  If your doctor isn't sure if the lump needs biopsying, s/he might send you to a breast specialist for an expert opinion.  I hope the lump turns out to be something that isn't serious.