Whichever car / taxi you take for visiting Sikkim, it is highly desirable to opt for a vehicle registered in Sikkim and not In some other state. So, please pay special attention to the vehicle’s number plate.
You can get shared cabs or personal cabs from Siliguri to Gangtok.
In Gangtok M.G Marg you will find many travel agencies that will arrange your permits for North Sikkim, Changu, Nathula Pass and Baba mandir. Make sure to carry 6 pictures and copies of your id cards
As soon as you get down at Siliguri, Taxi operators will be hovering around you for giving you a ride to either Gangtok or Darjeeling. Take a cab from Siliguri till Gangtok only. From Gangtok you can hire another cab(ask your hotel to arrange for a cab for best price) for sight seeing from Gangtok.