Music theory can be defined as the set of rules that govern how any kind of music, i.e., piano, vocal, composition, etc, is "suppose" to be written or performed. There are many aspects and nuances to music theory, and not all rules have to be followed at all times. On the contrary, most music theory rules are broken at one point or another! But it is important to understand the basics, especially when you first start to learn music!
Scales are any set of musical notes that are ordered by pitch. Most
people have heard the "do, re, mi..." sequence. That is a scale!
There are two types of primary scales, major and minor. Any scale can
be constructed from two basic rules of these scales. Scales move up
a sequence of whole and half steps. If you look at a piano, a whole step
is two half steps. Graphically, a half step is either moving from a white
key to the black key immediately next to it, or if there is not a black key
immediately to the right, a half step is the next closest white key. The sequence
to write a major scale is the following formula: W W H W W W H, where 'W'
is a whole step and 'H' is a half step. A minor scale is created using the
following formula: W H W W H W W. The image below shows what a whole
and half step are on a piano.
Musical chords are any combination of three or more simultaneously sounding notes. Chords are built off of a single note, the root. The most basic chord is called a triad. Triads are composed of a root, third, and fifth note. Similar to scales, major and minor triads follow a pattern. A major triad's root and third note are a major third apart (4 half steps) while the third and fifth are a minor third apart (3 half steps). Almost opposite of a major third, a minor third's root and third notes are a minor third apart while the third and fifth note are a major third apart. For example, a C major triad is composed of C - E - G where C is the root, E is the third, and G is the fifth. Why don't you try to make a triad?
Here are a list of pieces and their composers that are foundational to music that we know today:
Here is a playlist on Spotify that contains all of the pieces listed.