Sit down at the piano and place a page of sheet music and your guide to musical symbols on the music desk in front of you. Learn the notes. Find middle C, which is the most basic notes. Middle C appears on sheet music one line below the treble clef or one line above a bass clef staff (the five parallel lines). On the treble clef, one note above middle C is B; on the piano this is the white key immediately to the right of middle C. The scale continues in the same manner--each note up represents the next note on the piano.
On the bass clef, start with middle C, and go downwards on the sheet music and left on the piano.
Start reading at the upper left hand corner. Identify the clefs. A spiral form denotes a treble clef and denotes the higher-pitched octaves, while a slight curve with two dots is a bass clef. Determine the key signature. The key signature will appear as a series of lowercase "b" (flats) and symbols like "#" (sharps). On a piano, the black keys are sharps and flats--if you see a note with a "b" symbol next to it, the proper note is the black key immediately under that note.
Practice scales in the music's key signature. If a particular note appears on the upper left as a flat or sharp note, than that note will always be flat until the key signature changes or if the note is marked by a natural sign (sort of a parallelogram-type shape).
Identify the time signature. Immediately to the right of the key signature will appear two numbers, one over the other. The bottom number signifies the basic pulse of the movement--with 3/4 time the basic note is the quarter-note. The top number represents how many of these notes appear in each measure.
Practice and practice again. Learning sheet music is like learning how to read letters--you must keep working on it until it's natural and easy.