how can i start learning theory and optimizing my writing skills?

ok so i’ve been playing guitar for about 5 years now, but i feel like i really don’t even know how to use my instrument. I can play whole songs all the way through, but im seeking a better understanding of my instrument. can someone point me in the right direction? i guess on where i can start learning theory, scales, and the notes on my fret board and how they work together in and out? where do i even start?