So I was messing around with my acoustic guitar when I came across the following chord progression: G/D/Cadd9/Em. I really liked the way it sounded in that specific strumming pattern so I started thinking about some lyrics and I think I got myself a really solid chorus.

My problem is I don't understand "anything" about musical theory, as I'm still learning, so could somebody please tell me what other chords could I use in order to put togheter a solid verse and bridge?

I don't want progressions or anything, just other chords I can work with. If you can help me I would apreciate it...
That looks like G major, so try G Am Bm C D Em F#dim. Those are the triads in G major.
My name is Andy
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I don't know so much about theory too, but I can suggest to try for example Em7 and Dsus2, they will sound good in this progression.
F#diminished would be a great chord to use inbetween Em and G. It resolves well and sounds original. You could also use a D in between Em and G, which also resolves well. Am6 could also work. C would also work, but no were near as well as F#dim, D or C.
Just try it and see which sounds the most "finished" to you.

Also, what are you doing with your singing melody? Remember, that's where the magic happens. What the guitar plays isn't anywhere as important to what the singing (or any other instrument, if you're writing an instrumental) melody is doing. Guitar is (usually) just an accompaniment instrument.