Clef
Alto clef notes
The five lines of the alto clef are F, A, C, E and G, from the bottom up, and the four spaces are G, B, D and F. The alto clef is a C clef: the point where its two curves meet marks middle C, and in the alto clef that is the middle line.
Read it, then play it
Play the note you see on the staff. The exact key counts: how high the note sits tells you which octave.
The lines: F A C E G
From the bottom up, the lines are F, A, C, E and G. There is no standard rhyme; violists learn middle C on the middle line and count from there. Each line is a third above the one below it, so you are really skipping every other note name.
The spaces: G B D F
From the bottom up, the spaces are G, B, D and F. Read them from the lines around them: the space above middle C is D, the one below is B.
Read from landmarks, not note by note
Start from the note the clef marks: C on the third line. From there, every line or space up is the next note name and every one down is the note before. Middle C sits on the third line.
Above and below the staff
Notes that don't fit on the five lines get short ledger lines. With one ledger line either side, the alto clef covers D3 to B4: 13 white keys on the piano.
How to read ledger lines