reading sharps

It's a KEY SIGNATURE (or part of one). It refers to all notes on whichever line or space it's on PLUS all octaves of that note above or below.

For example, a commonly used key signature is a single sharp placed at the beginning, on the top line of the treble staff, which is the note F. It means ALL F notes are to be played as F sharp. If anywhere in the music the composer wants a natural F, a natural sign will be placed just before the note. It temporarily cancels that original sharp sign for one measure. In the next measure all F notes become sharp again.

The single sharp key signature is associated with the keys of G major and E minor. Both of those keys use the note F sharp instead of F so it makes sense to just put that sharp sign at the beginning instead of in front of every F note in the music.

I've posted this before but it bears repeating.

Here's a table some have found useful.
Major Relative Key I ii iii IV V vi vii°
Key Minor Signature 1 2 3 4 5 6 7
----------------------------------------------------------------
C major A minor C D E F G A B
G major E minor # G A B C D E F#
D major B minor ## D E F# G A B C#
A major F# minor ### A B C# D E F# G#
E major C# minor #### E F# G# A B C# D#
B major G# minor ##### B C# D# E F# G# A#
F# major D# minor ###### F# G# A# B C# D# E#
F major D minor b F G A Bb C D E
Bb major G minor bb Bb C D Eb F G A
Eb major C minor bbb Eb F G Ab Bb C D
Ab major F minor bbbb Ab Bb C Db Eb F G
Db major Bb minor bbbbb Db Eb F Gb Ab Bb C
Gb major Eb minor bbbbbb Gb Ab Bb Cb Db Eb F
----------------------------------------------------------------

Note that Gb and F# are enharmonic, but they contain the same number of sharps or flats (6) so one can use one or the other.