This is the outline of a basic open G chord (green) and an open C chord (red). If your music says G for the chord, you could play the green notes, if the music says C, you could play the red notes.
You can also try singing the notes, Do, Re, Mi, etc., as you play.
Use this play-along to try out the I and IV chords. https://www.youtube.com/watch?v=o5xCVtkOPVc. When the V chord comes around, try playing different notes to find out what sounds good. (Hint: So, Ti, Re sound very consonant (they don’t clash) on the V chord.)
