How to say "I live at the bottom of a hill"?
山, in general, refers to a mountain or a hill.
  
  坡 refers to a slope.
  
  Therefore, 山坡 refers to the side (or slope) of a hill (or mountain).
  
  脚 refers to the foot.
  
  So, 山脚 refers to the foot of a hill (or mountain).


Depending on what bike you are referring to, it could be a motorcycle (摩托车) or bicycle (自行车).

I think the translation provided by Google is okay:


  Because I live at the bottom of a hill it is hard to ride my bike to
  work.
  
  因为我住在山脚下，骑自行车(or 摩托车)上班很难。


If you are staying in a valley, 山谷 will be more appropriate.