I'm about 5"6, weigh approx 170 and my breast are large 40G. What do I need to do to get a reduction?

Thank you for the question. Based on your description you may be an excellent candidate for breast reduction surgery.    
“Typical” patients who present for breast reduction surgery are women who have disproportionately large breasts, causing problems such as neck/back/shoulder discomfort, postural changes, bra strap grooving, skin irritation/rashes under the breasts, and/or difficulty with activities of daily living and/or exercise etc. There may be both physical as well as psychosocial “stress” caused by the disproportionately large breasts.
Reducing breast tissue mass and elevating the breasts on the chest wall tend to improve or alleviate many of the symptoms associated with the disproportionately large breasts.
Patients considering breast reduction surgery should also consider the potential downsides (risks/complications) associated with the procedure as well.. Poor scarring, for example may be associated with the procedure.  Additional surgery may be necessary in the short or longer term for a multitude of reasons.
When the time is right, I would suggest starting with the American Society of Plastic Surgery and/or the Aesthetic Society of Plastic Surgery to obtain a list of well experienced board-certified plastic surgeons.
Then, I would suggest you visit a few surgeons whose practices concentrate on aesthetic surgery. Ask to see lots of examples of their work and preferably speak/see patients who have had similar procedures done.
Then, it will be important that you communicate your goals clearly with your plastic surgeon and make sure that you have realistic expectations prior to proceeding with any type of surgery. In regards to breast size desired, also communicate carefully. In my practice the use of goal pictures are helpful in this regard; a discussion of cup size can be confusing and imprecise.   Therefore, I would not suggest that you communicate your goals and/or base your satisfaction with the outcome of surgery on achieving a specific cup size.  I also find that the use of pictures is more helpful than the words “natural” or "proportionate” etc., which can mean different things to different people.Best to check directly with your insurance company to see whether breast reduction surgery is a “covered” benefit with your specific policy. Then, given that every insurance company has different “criteria”, best to identify exactly what your specific insurance company considers criteria for approval. Generally, the more documentation you have (for example, from your primary care doctor, physical therapist, chiropractor etc.) the better when it comes to obtaining insurance “authorization” for the procedure. Again, your insurance company will be able to be more specific when it comes to what "hoops" they want you to jump through. Also, for many insurance companies, the closer you are to your long-term ideal weight, the better.
This documentation and letter/pictures from your plastic surgeon will help you obtain authorization.  
Also, prepare to be persistent when dealing with health insurance companies.
 I hope this helps.

Hi dear.. Thank you for your question!
It is hard to assess without first seeing you in person, I'd recommend having a face to face consult with a board certified plastic surgeon who can evaluate your case more closely and provide you the possible solutions for your situation as well. If you don't feel comfortable with the previous responses, I'd suggest seeking for other opinions from distinct plastic surgeons and express them your concerns.
I'd suggest looking up for opportunities in countries like the Dominican Republic. Here we count with a wide variety of board certified plastic surgeons who can assist you in this journey and offer you affordable options without putting your health at risk at any time.
Hope this helps.
Kind regards,
Dr. Jiménez Toribio 
Plastic and Reconstructive Surgeon.

Hi and welcome to our forum!
Based upon your history of neck / back pain, bra strap grooving, rashes beneath the breasts (intertrigo), and your breast size, you would be considered a suitable candidate if you are in good health, not morbidly obese, and have realistic goals.
In addition, although it has recently become more difficult to obtain insurance coverage for the procedure, it is worth the "red tape", as I think this would be a covered medically necessary procedure in your specific situation.
Seek a consultation with a board certified plastic surgeon.
Best wishes...