What must I do to get rid of dark inner thighs?

Get evaluated. Various reasons for skin discoloration. Infections, irritation from clothing, cosmetic or soaps, or discoloration from the aftermath of what might have been there.Sometimes changes in the skin from friction can contribute to appearance. Acanthosis nigricans is also possible, this condition can be associated with underlying medical conditions. It's best to go to a dermatologist to be evaluated.
Dark inner thighs… May be acanthosis nigricans, a skin condition that is sometimes associated with Insulin resistance and diabetes. Here is a medical website that goes into detail about this condition: http://www.Ncbi.Nlm.Nih.Gov/pubmedhealth/pmh0001855/.