You can use Tuk Tuk (Tuk Tuk is typical Thailand rickshaw) or Taxi, but I usually prefer travel by boat throw Chao Phraya River - It’s the most cheaper way to go there, and it’s also interesting, because there is lot of Thai people who travel to work.
You can go by foot from Khao San road to Santichai Prakan Public Park (Suan Santichai Prakan) - it took only a few minutes. There is Phra Athit Express Boat Pier (Pier number 13). There is no reason use “Tourist boat”, you can use ordinary lines. Wat Pho and Wat Arun are near Tha Tien Pier (Pier number 8).
Between boat pier and Wat Pho is Tha Tian Market - there is dried fishes.
If you are going from Tha Tian Market down throw Maha Rat Road past Wat Pho walls you can bought Thai ointments (like Tiger Balm) and many kinds of massage oils.