Learning what chords to use for substitutions can be hard. I made this chord chart that should give you a good place to start. It includes the most commonly used substitutions for C Major, minor, and 7th. For example, if you are looking for a chord to replace a simple “C”, look under the “Major” section of the chart. All the “Major” chords will work with C depending on the song. The more commonly used are first. Most of the time when you are backing someone up you can substitute with the top chord in the column. The three voicings that you see below can be used as melody chords (see below). The “Minor” and “7th” chords are used the same way, though there are not as many to choose from.
I like to use substitution chords to fill out a melody. Since the highest note of any chord is going to stick out most, you can just play a sub chord that hits the melody note. This is great if you are playing solo, it fills out the song, as you end up playing harmony as well as the melody.