You should try to work from sheet music as much as possible. Nearly all sheet music is arranged for piano and this presents problems for guitarists. Piano music uses three staves thus:
A guitar player need only look at the top stave, which contains the melody line (the tune), the lyrics and the chords. Sheet music does not indicate what rhythm the guitarist should use. This is where your creativity and background of rhythm playing (i.e., rhythms and techniques outlined in this book etc.) must be put to use. You should generally only attempt songs that you know well, and listen to original recordings of these songs to gain rhythm ideas.