Learn the treble and bass clefs. The sheet music you will use depends on the instrument you are using. For instruments with a higher range such as the flute or clarinet, you will be using the treble clef. For instruments with lower range such as the baritone or tuba, you will be using the bass clef. Piano uses both bass and treble clefs. Memorize both clefs.
Learn and identify the symbols in sheet music. You will need to know that a "#" means sharp and a "b" means flat. You will need to know the symbols for rests and each note. You will need to know what a treble and bass clef looks like, as well as the meaning of time signatures. You can easily find resources for symbols online.
Learn the names of the series of 12 notes in a chromatic scale. You can read each clef like an alphabet starting from A and going to G. Every note has a sharp and a flat except for B, C, E and F. The flat of one note is also the sharp of the previous note.
Learn the time and key signatures. The time signature is at the beginning of each staff. This tells how many beats in a measure and which note gets one beat. Practice the time signature. The key signature is also at the beginning of the staff and shows how many sharps and flats are in the song.
Learn the differences with the bass and treble clef. The bass and treble clef are nearly identical except for the starting points of the notes. The bottom space of a treble clef is an F, while the bottom space of a bass clef is an A.
Practice. Once you learn the notes and their functions, the only thing left to do is practice until it becomes second nature. There are plenty of online resources that will help you train and drill music notes. You may also want to seek help from an instructor who can go into further depth on music theory.