Most of the sheet music you will buy in a music store will be arranged for piano. Piano music is written using two or three staves, with the chord symbols written above the top staff. It may also contain unfamiliar symbols and terms. At this stage you need only look at the top staff, which contains the melody line (tune), the lyrics and the chords. In some sheet music chord diagrams may also be included.
As most sheet music is arranged by keyboard players quite often the guitar chord shapes given are incorrect, unnecessary or impractical, and many piano arrangements of guitar based songs do not sound anything like the recorded version. Guitar tablature versions of sheet music are gradually becoming more popular and in many cases are very accurate arrangements of the song. If you want to learn more about reading music and tablature see Progressive Guitar Method Book 1: Beginner TAB version.
Also many piano arrangements are in difficult keys for a beginning guitarist and quite often use unnecessary chords. Piano sheet music also gives no indication of how to strum the chords. So piano sheet music is only a guide for a guitarist but is useful for lyrics and a general chord guide.
If the song contains chords that you are not familiar with you can:
Use the easy chord table which lists the type of chord you may see in the sheet music (on the left of the table) and the simpler chord you can substitute (on the right of the table). If you know how to transpose and substitute chords you can play almost every song ever written using only a few basic chord shapes. It is recommended that you do further chord study and at least learn how to play bar chords. See Progressive Guitar Method: Bar Chords.