Thanks for the reply. But what's with those people who say they don't really know any music theory yet they can play in the band with others? Are they just playing by ear and try out different chords which might sound somewhat good to make a harmony?
You've asked two different questions:
PLAYING doesn't require music theory, though knowing it helps.
COMPOSING or arranging requires a pretty deep knowledge of music theory (yes, I know there are composers who aren't theory savvy, but they're the minority).
What you need to do is sit down, take a deep breath, and start at the beginning. If you want to play guitar, start with the beginners course and go through each lesson - some may be review and you'll breeze through, but I've found very useful tidbits tucked away in a lot of his lessons that true beginners probably miss if they never watch them again.
One of the Very Cool(tm) things that Justin's course does is provide actual metrics for you to see when it's time to move on to the next stage. When you can do so many chord changes a minute, and know this or that strumming pattern, or whatever else is in that stage: don't forget the ear training exercises, it's time to move on.
If you can do most of it but are stuck on one or two things, then practice those things like mad until you can do them, then move on.
If you're serious about learning to compose music, you've got to study music theory. You can do this concurrently with your playing, because honestly the two don't have a whole lot to do with each other at the beginning playing level, especially with guitar. Piano/keyboard is less forgiving in this regard.
Get a good, solid book. You can buy the old (2003) edition of Tonal Harmony on Amazon for under twenty bucks used, $45 new (the workbook is more expensive, but still a lot cheaper than the latest edition). That's a steal - the latest edition is over $150 new for just the text, though you can buy it packaged with the workbook at some reasonable prices through 3rd party sellers. In any case I can guarantee music theory hasn't changed that much over the past ten years.
How much of it you use will be up to you and how you choose to compose and arrange your music. There are metal bands that throw long 'suspension' chains in their music like something out of Bach or Hayden. Others just get by with knowing a few chord progressions and scales.
Oh, and learning theory is hard work. There's a LOT more to it than learning chord progressions and scale degrees. Only you can decide if you want to go down this route.
My suggestion is to focus on your playing for now - work especially on your TIMING. Do everything to a metronome, this way when you get the opportunity to play with other people, you'll be able to play IN TIME. Without that, jam sessions quickly become train wrecks.