Based on your description, it appears that you have acanthosis nigricans. These conditions have a strong relation to one's internal health and it needs to be evaluated as often quoted - Skin is the mirror of internal health. Attempting treatment without understanding the pathology will be futile and make the course longer. Your dermatologist will decide a treatment plan for you after a consultation which probably will include a retinol based night cream and a vitamin C based lotion.
Sun protection is mandatory and will require usage of Sunscreens. You may benefit from usage of a laser device i.e. Q- switched Nd YAG laser but that is in addition to the main treatment. Internal health needs to be fixed based on blood chemistry reports.
Your dermatologist will decide a treatment plan for you after a consultation.
Apart from active pharmaceutical medicines, a healthy diet and lifestyle is mandatory to achieve results.