Major and minor add9 chords are formed by simply adding another scale tone (note) to the existing chord. You can add any note of the scale of the key you are playing in to any of the chords in that key.
Each time you do this, it will change the name of the chord, but knowing all these names is not essential for now. The best thing to do is experiment and memorize the notes and chords which you think sound good together. This example is in the key of E minor and contains various scale tone additions and substitutions. Learn it and then use the ideas to make up your own progressions. With the knowledge you now have, you will be able to play both melodies and accompaniments for hundreds of songs and instrumental pieces, and you will also be able to write your own.