If you are going up the neck of the guitar (that is up in pitch, starting from the nut and finishing at the pickups or sound hole) you would use SHARPS, as shown below.

ON MOST ELECTRIC & ACOUSTIC GUITARS THERE WILL BE
DOTS OR MARKS ON THE FRETS THAT ARE SHOWN BELOW.

As you can see below, between all the notes that have a tone move between them there is an ACCIDENTAL (Sharp or Flat). B & C and E & F only have a semitone move between them so there is no such thing as a B sharp or a C flat or E sharp and F flat.