Hello.
For a very long period of time I wanted to share my own feeling of what a music theory is and why do we need it.
So this post is about basics and application of some rules into play.
Music Theory — why do we need it?
Learning some pieces of music theory is very essential if you want to become a real musician.
Maybe you've heard some stories about people who have never studied music theory and somehow became great musicians — but don't believe them. At least all that musicians could play one instrument or maybe few — and their teachers explained them all basics — it's just inevitable. Following some rules helps to feel when music sounds right and when it sounds wrong. Sometimes it looks like maths but our world is full of maths — only take a look at nautilus or sunflower with its Fibonacci sequence embodied in their shapes.
Notes and Chords
We have twelve notes in our music. Hell knows why only twelve but not 34, but it's enough to create stunning music.
Here they are:
A, A#, B, C, C#, D, D#, E, F, F#, G, G#
Looks not very friendly, right?
But let's take a look at it on a piano:

Now it look pretty simple, isn't it?
But I probably wanna ask — Why does it start with «C»?
I'm not sure about it but I gues that's because C maj is the easiest scale on Earth.
Thousands of popular hits are made on that scale ang composed from easy senquence of chords which includes notef from C maj scale.
Take a look at the picture and try to play these notes together:
C-E-G — that is C chord
A-C-E — that is Am chord
F-A-C — that is F chord
G-B-D — that is G chord
C, Am, F, G — sounds good, isn't it?
C, F, G, C — not bad also.
Most of the songs are basing on that simple chords.
Try to play that 4 chords with left hand and use your right hand to play any white keys — they will all sound good. That's because you play chords from C scale in left hand and notes from the same scale in right. Together they sound harmoniously.
So every scale consist of notes which can be taken together to create chords. When you play chords and notes from one scale your music will be right.
I think that's enough for today. Next time I'm going to tell you more about scales and their patterns.
But for now — C scale is perfect to begin with. So you better try and hear it.
Good luck!