How can i get rid of dark under arms and or inner thighs?

Depends on cause: Hydroquinone based creams are used to help lighten dark spots. What you are describing, however, could be a condition called acanthosis nigricans. This is commonly associated with high Insulin levels and Insulin resistance. I see it frequently in patients who are significantly overweight. If that is what you have, it needs to be addressed by your doctor.