Purchase a piece of sheet music. It is best to find sheet music for a particular melody or song that you like, so you know what the symbols, notes and chords are supposed to sound like. Study this piece of music closely.
Look at the left end of the staff (the five lines on which the musical notation is written). The very first symbol (the clef) tells you what line represents what note. The G (treble) clef and the F (bass) clef are the most common clefs, with the G clef on top. The G clef reads, from bottom line to top line, E, F, G, A, B, C, D, E, F, and the F clef reads G, A, B, C, D, E, F, G, A.
Find the time signature. This is the fraction that comes immediately after the clef marker. The top number tells you how many beats are in a measure, while the bottom number tells you what note gets the beat. A four in the bottom number means that a quarter note gets the beat, an eight means an eighth note gets the beat and so on.
Look to the right of the time signature. Sometimes a series of italicized lower case Bs or italicized number signs called a key signature appear on the lines and spaces before the first note. These symbols tell you that every time you play a particular note (assuming there is no sign---called an accidental---telling you otherwise), it is to be flat (lowercase Bs) or sharp (number sign).
Examine the notes on the sheet music. You will notice that some have stems and flags sticking out of them. These stems and flags tell you how long a note is to be played. An open circle is called a whole note, and it gets played for an entire measure; a blackened circle is a half note, and it is played for half a measure; a blackened circle with a stem is a quarter note, and it is played for a quarter of a measure; a blackened circle with a stem and a flag is an eighth note and it gets played for an eighth of a measure. Further subdivisions are marked by more flags.
Find a chord in the sheet music. Chords are marked by notes stacked on top of each other. To play a chord, you simply have to hit each of vertical notes at the same time. Duration of chords is marked in the same manner as duration of single notes.