What does the note start on for the key of a flat major?

I'm not sure your question is clear enough to answer.

Are you asking "What is the 1st note in a major scale
of a flat key?

As in, what's the 1st note in a Bb major scale, or an Eb
major scale?

If so . . . You can start on any note of a major scale. It's
still that same major scale. In a playing situation, starting
on a different note, and playing the scale could be playing
in a mode.

In general, the first note in ANY key would be the name of
the key. Key of Bb, 1st note is a Bb.

Each note in a major scale is referred to as a degree. In a
C major scale, C is the root, or 1st degree. D is the 2nd
degree, etc. It doesn't matter if it's a Natural key like C,
or a flat key like Bb, or a sharp key like C#.

If your question is . . . "What key am I in if I play a flat major
scale but don't start on the 1st note?" then that's a different
question all together.

There isn't a short answer to this question. Because now
you're specifically dealing with modes. The key your in,
as in, the ROOT key will be the same, regardless what note
you start on. But . . . what MODE you're in depends on what
note you start on.

There are 7 modes. Look up MODES on the net. Each degree
of the scale can be your starting point while playing the scale.

Also look up "how to build a major chord scale" or "how to build
a major harmony scale."

You'll find that a major chord scale has 7 chords, one for each
note in the major scale.

The 7 chords, starting from the root, in the key of C for example, will be:
CM, Dmin, Emin, FM, G7, Amin, Bdim, back to the root, or tonic, CM.

If you start playing your major scale from the 2nd degree, for instance,
over some chord progressions, you'd be playing in D Dorian Mode.

This is a really simplified explanation. It's a bit more involved than that.
But you should get the gist of it.

If not, drop me an email at "wayne at thewaynereedconnection.com"

Wayne