that‘s a big topic I guess.
I‘ve studied a lot of theory but all of my books are in german, so that wouldn‘t be helpfull.
First of all you have to answer what‘s your goal?
In my opinion you have to learn at first what is triad and common intervalls.
There are a bunch of good tutorials on yt. Go and check out stuff from Jeff Schneider and Aimee Nolte.
On the other hand you can open up a theory thread here on proguitar. While you are learning you can ask specific questions and we can help you to develop a good theory background.
Thanks for the info! My little brother is learning acoustic guitar. Until now, he has learned different techniques like strummer, flatpicking or travis picking. His music lesson is going well under the trained professional and hope so he learns well and play beautifully!