You can take a taxi from many different locations, including Rishikesh or Dehra Dun. If you take a taxi from Dehra Dun the fare should cost you somewhere around 400 rupees, while if you take a taxi from Rishikesh it will cost you around 1,100 rupees. You can also hire a jeep from Uttarashi, which costs 2,500 rupees. You can also take a taxi, which is shared with multiple people from Dehra Dun and costs around 50 rupees per person.
You can also take a bus from Dehra Dun, which is the main stopping place for all buses in the area. The trip will take around 1 1/2 hours and cost you around 28 rupees. You should inquire for buses at either the Library bus stand or the Picture Palace bus stand. However, if you want to take the back roads that is possible as well. To take the back roads you should go to the Library bus stand and look for the buses that head to Yamunotri or Sankri. The buses to Yamunotri take around 6 hours and will cost 174 rupees per person, while the bus to Sankri takes 7 hours and costs 120 rupees per person.