When Chords are laid out in normal sheet music form, the main focus is usually on the melody of the song. This is another case where knowing how to sight read is useful.
The example above is in 2/4 time. This means that there is 2 beats to the bar, or 2 down strums every bar. You can use the melody to help you work out how to strum the song. The 1st chord is "D" and the 2nd is "Em", the Eminor chord is place above a quarter note, that quarter note falls on the 2nd half beat. When you strum on a half beat you strum up. On the 3rd bar there is a "D" and "A7" chord. Both these chords are placed above quarter notes. For this bar you will strum down once on the "D" and once on the "A7".
Here is the strum pattern for the song above.
Here's how you could make the pattern a little more interesting.
A major percentage of sheet music is written for the piano and vocals unless otherwise specified on the front cover. When buying sheet music for the guitar, make sure you buy a guitar version. This is generally shown on the front cover of the music as (piano,vocal, guitar) or (tab version).