How am I supposed to know what string(s) to strum just by reading chords? I barely started playing guitar and I'm confused.
The strings to be strummed are determined by which cord you are playing and what position the chord is in.
I am assuming that since you are a beginner, you are playing open chords. You need to learn the chords and be especially aware of how many strings are included in the particular chord and then strum only those strings.