I put "ears" in snarky quotations because it's really a combination of listening to the music while your own "inner voice" speaks out along with what’s being played. Groovy sh-t. Listening is key because otherwise you're just masturbating all over your guitar while everyone else is trying to play music and nobody wants to see/hear that. People who masturbate in public typically don't last long in bands. I don't know if that’s statistically true but I would imagine it is, so for the good of learning we'll assume that I'm telling you the truth.
Anyone can listen to a melody that's being played over a different piece of music and tell if it "fits" or not. You can tell if a note sounds strong, or fair, or weak, or absolutely terrible over a specific chord even if you don't know a thing about music. That's basically how I taught myself to play guitar when I was younger. I knew one shape of the minor pentatonic scale along with one shape of the natural/harmonic minor scale and that was it as far as any "knowledge" of theory went. I'm sure this sounds familiar to most of you. I'd sit down in my room covered in posters and I'd noodle on my guitar playing along with bands like Blue Oyster Cult and Black Sabbath. I'd find the root note and bullsh-t on a combination of the few scale shapes I knew as well as random licks I'd learned from magazines or other people's solos.
Eventually I noticed that some, or more often than not, all of the notes I was playing sounded horrendous and I would then have to use my ears to quickly bend the note in tune or find my way there in some sort of chromatic fashion before anyone noticed it sounded like shit. The longer I played the better I got at this and it got to the point where I could play fairly well to most music and have it sound more or less like I knew what I was doing. Whether I did or didn't is irrelevant to the point. Obviously I had NO IDEA what I was playing; I just knew what sounded good and had been playing/training my ears long enough to get by without making anyone cringe. Apparently some very good musicians get by their whole careers with this approach and although I feel having a good "ear" is crucial to being a musician, knowing the technical aspect of music is just as important.
As soon as I learned theory everything changed. Because I'd been playing on my own for so long I recognized the sound and feeling of scales/modes almost immediately and it pieced everything together for me and gave everything a name. I finally knew what I was doing and once that happened I could combine my already trained ears with my newfound knowledge of theory. The difference in my playing was night and day. I still use my "ears" to play but now I have a stronger foundation to work with as well as unlimited options to approach musically over chord progressions.
My point to all of this is that in order to be a good player, no matter what skill level you're at, no matter what you play, if you don't know theory you should start learning it today. There are online lessons everywhere as well as books (don't forget those) that will teach you everything you need to know. Also you should be training your ears in any way possible. Even listening to music, in general, is a form of ear training. Playing along to my favorite bands helps me immensely because it keeps me out of the horrible box of doing finger exercises to a metronome which eventually taps a hole into your brain. It's fun and it keeps me on my toes because there’s a different key/time signature/tempo for me to work with in each song.
I'm not saying this is the only or best way to do it. I'm just telling you how I did it, and I hope it works for you or gives you some idea of how to make this fun for you, because at the heart of it that's what's most important!