The best month to travel are mid September throuch March. Sikkim can be reached from India by air from Delhi or Calcutta to Bagdogra, or overland from Nepal or Bhutan. Sikkim requires a visa for Indian and special permit for Sikkim which can be obtained from the India High Commission office from your home country. It is possible to exchange foreign currencies only in the banks. Indian currency is easily accepted here.
Credit cards are difficult to use.