Learn the grand music staff (stave) with both the treble and bass clefs. Both with be used in a full score. The music staff is comprised of five lines and four spaces, each representing a note. The notes on the spaces, starting with the first space, are F, A, C, E for the treble clef, and A, C, E, G for the bass clef. The lines are E, G, B, D, F for the treble clef, and G, B, D, F and A for the bass clef. Making up phrases for the lines and spaces can help you memorize them. For instance, "All Cars Eat Gas" for the spaces on the bass cleft, and "Every Good Boy Does Fine" for the lines on the treble clef.
Familiarize yourself with major and minor scales. You can use a scale chart that lists the major and minor scales by key. Obtain these scale charts at a local music store, or use a free online source (see Resources).
Learn basic music notation and embellishment. Musical notation is placed on the staff with a bass or treble clef notation, a key signature (the key the song is in), a time signature indicating how many beats are played per bar of music, and a combination of note values representing how long notes are held out and placed within a bar until they equal the proper number of beats. Basic musical notation can take some time to learn, but understanding basic note reading will allow you to look over a score and understand what you're seeing. You can gain a full understanding of musical notation with a beginning theory book from a local music store or at a free website (see Resources). All note values, such as a whole note, half note and eighth note, have corresponding rests of equal value that you should learn as well.
Read as many musical scores as possible. Alternate between single and multi-instrument charts. Listen to a recording of the sheet music when possible. Try to follow along and understand how the notation indicates the recording. Listening will also help improve your sight-reading skills. Practice making your own music scores as well. The more you immerse yourself in music notation, the better your musical reading and writing skills become.