Actually, it usually is the chord you start on that determines the key, at least in the blues, the key word being "usually." There are obvious exceptions.
Starting "on the five" means you're starting at the tail end of a (non-existant) sequence, where you start with a measure of V, a measure of IV and then back to the I (and any turn around in the song) before starting at the beginning of a 12 bar progression.
It's a lot of work to explain what is in essence a very simple concept.
What you're talking about with the Do, Re, Mi, stuff is the major scale.
Based on the music teacher site you saw, you may already know your keys are arranged from C, which is no flats and no sharps.
They go in exactly this order for "sharp" keys.
- C - No sharps
- G - One sharp, which is F#
- D - Two sharps, which are F# and C#
- A - Three sharps, which are F#, C#, and G#
- E - Four sharps, which are F#, C#, G# and D#
- B - Five sharps, which are F#, C#, G#, D# and A#
- F# - Six sharps, F#, C#, G#, D# and A#, and G#
You might start to notice a pattern here.
"Flat" keys:
- C - No flats
- F - One Flat, Bb
- Bb - Two flats, Bb and Eb
- Eb - Three flats, Bb, Eb, Ab
- Ab - Four Flats, Bb, Eb, Ab, Db
- Db - Five Flats, Bb, Eb, Ab, Db, Gb
- Gb - Six Flats, Bb, Eb, Ab, Db, Gb, Cb (which is actually B)
Arrange that a little differently and the pattern becomes a little more obvious.
- Gb - Six Flats, Bb, Eb, Ab, Db, Gb, Cb (which is actually B)
- Db - Five Flats, Bb, Eb, Ab, Db, Gb
- Ab - Four Flats, Bb, Eb, Ab, Db
- Eb - Three flats, Bb, Eb, Ab
- Bb - Two flats, Bb and Eb
- F - One Flat, Bb
- C - No sharps
- G - One sharp, F#
- D - Two sharps, F# and C#
- A - Three sharps, F#, C#, and G#
- E - Four sharps, F#, C#, G# and D#
- B - Five sharps, F#, C#, G#, D# and A#
- F# - Six sharps, F#, C#, G#, D# and A# and E# (which is actually F)
The keys of Gb and F# are actually the same key, but written in a different notation.
Gb Major scale: | Gb | Ab | Bb | Cb(B) | Db | Eb | F | Gb |
F# Major scale: | F# | G# | A# | B | C# | D# | E#(F) | F# |
This still doesn't tell you how to tell a major key from a minor key, but you can always tell your 1-4-5 from this list by picking your 1. The way this is arranged, your 4 will always be directly above the 1 and the 5 will always be directly below the 1.