Welcome to music theory. I'll teach you everything you'll learn in your three years of Music Theory (though I suspect four-year Music Theory students will spawn in the near future) with the Timster.
I'm going to start off with this little tidbit you'll learn more toward the end of your time in Music Theory: A, or A4, or middle A, is tuned to 440 Hertz. 440 Hertz means that the sound wavelength oscillates 440 times per second, and if you're familiar with pitch theory or elementary physics, higher sounds have higher frequencies.
The one rule you'll learn is that the frequency of a note an octave above a particular note will have two times its frequency. For example:
A4 has a frequency of 440 Hertz.
A5 has a frequency of 880 Hertz, and A3 has a frequency of 220 Hertz.
None of that is actually important yet, but it's nice to know that when we talk about how pitches have changed over time. Before the Baroque era, there wasn't really any such thing as a standard tuning for instruments. We know, however, that the Baroque era brought us standard tuning because of Bach (pronounced "batch") and his Well-Tempered Clavier, a set of compositions that goes through the 24 different keys that arose from standardizing pitch. Allegedly, people during the Baroque era tuned their instruments to 415 Hz, making A#4 about 440 Hz instead.
And no, that's not just random information. You'll be doing a history of music unit in your last year of Music Theory so be prepared. I don't think we ever successfully finished that unit but you—as a four-year Music Theory student—must keep this in mind.
So with the arrival of pitch standardizing, music notation and the 12-tone scale became a very real thing—real enough that just about all Western music is based on that same scale.
Here I show it in C major: