Hi Tug and welcome,

If I might add a bit more to what Christopher and Tod have already said, music is about structure and your 12 bar blues is a perfect example. Countless songs use the structure of a 12 bar setup. It is very common that a song will follow a 1 - 4 - 5 progression for those 12 bars. 

An example:

Key of C Major is made up of the following chords: 
C major - Dminor - Eminor - Fmajor - Gmajor - Aminor - Bdiminished

The 1st - 4th and 5th chords in this key are: C major - F major and G major. 

You could make up your own little song using these 3 chords and it would sound pretty good. You can mix and match them as you please remembering that at the 12th bar you repeat. And not to get too far into the weeds just yet, after a few repeats of your 12 bar structure you might want to put in a "bridge". Then get back to your 12 bar pattern. 

The courses and lessons here are to help each of us understand the structure of music. Once you have some basic concepts down you will be begin to understand how chords and scales fit together and why the term "key" is so important.