A Note on any instrument and the Guitar can be either of Three:

Natural - Not Sharp or Flat
Sharp   - Up One Fret 
Flat    - Down One Fret 

  You would not Combine Sharps and Flats and each Letter is used only once, Any Note can be Sharp Or Flat - It depends on What Key you are in, The Natural State (No sharps or Flats) is the Key of C  

Mouse over the Buttons to See All Sharps or Flats

See the Printable Version