@Rozay Oro
To start there are resources from traditional music learning avenues for getting your bearings with Music theory.
I'd suggest picking up a basic music theory book & a jazz music theory book for when you start moving
into more advanced harmony.
My personal picks would be this series of berklee books:
Berklee Music Theory Book 1
Berklee Music Theory Book 2
The Berklee Book of Jazz Harmony
Jai Josefs Writing Music For Hit Songs
I don't personally work from the berklee texts but they should be a good jump off point to get
the basics under your finger tips. They should cover how to construct
Note Lengths
Rest Lengths
Intervals
Triads - Major, Minor, Augmented and Diminished
Inversions for Triads (Root, 1st Inversion, 2nd Inversion)
Sevenths - Major, Minor, Minor7b5, V7, etc.
And once you get to the Jazz Harmony text you'll most likely begin covering ii, V7, I and this is where you'll
get into chord functions and how they work within a chord progression:
Like how the
"Dominant" Chord wants to
"Resolve" and typically that means resolving to the
"Tonic" which is more stable.
However between the V7 chord and the I chord and back again you can take the listener on a journey.
Chords like the ii and the IV are
"Sub-Dominant" in function, allowing you to create for
"Tension" because these chords will
want to resolve much like their
"Dominant" big brother and this can allow for you to add harmonic variety as you travel back
to the V7 or to the I. While chords like iii and vi behave like the tonic does but they're considered "less stable" and can even be used
as chords to resolve to, this is where topics like
"Substitution by Function", come into play.
If you want an idea of how this sounds take your keyboard or guitar and play a V7 chord like
G7 - (Built like this G-B-D-F) and then play a C major chord (C-E-G-) you'll immediately hear how this sounds.
Most of my knowledge for this chord progressions and tension and resolution is very basic at the moment, so if there's
someone more learned on the topic that wants to correct me, feel free to jump in.
With that said, please pick up some texts and bust out the pencil & paper, it'll allow you to get a grasp on song writing
that is difficult to acquire using purely digital means, DAWS tend to make it easy to begin "writing" but the teaching methods
on offer on youtube are often pretty terrible.
These texts will offer many opportunities to not only READ the material but APPLY it.
APPLICATION is KING, followed closely by ANALYSIS .
By analysis, I mean listening to songs and trying to pick out what's going on within the record.
How long are the chords being held ? What note lengths are the hi-hats or Kick & Snare being played at ?
What kinds of chords am I hearing ? etc.
Do these two things and your ability to understand will improve by leaps and bounds, I know mines did and I'm still learning !