Learn the basic layout of traditional musical notation on a staff, or the five horizontal lines that notes are written on. You should be able to quickly decipher what a note on a given line means and also what timing the various shapes of each note such as a half-note, quarter-note or eighth note means.
Take a look at the timing that's indicated by two numbers, one on top of the other, at the left-most part of the staff. Then look at the notes indicated to the left of the timing notation to decipher, on the basis of which of the horizontal lines are marked, what key the song is to be played in.
Use the notes that come after (to the right of) the timing indicator to play the melody of the song. Keep in mind that you need to read both the note and the timing of the note (indicated by its shape) to properly use the lead sheet.
Check out the chord symbols that are written above the staff so you and other players know what the chord progression of the song is without having to look at the notes. Chord progressions are usually written as a letter or sometimes a number and letter such as "D7."
Find the lyrics of the song underneath the lead sheet's staff and look at the placement of each syllable of a lyric in relation to a note to know what note a particular lyric should be sung at and at what tempo.