Harmonicat: More likely you'll want to play popular music in some genre, and not that much is specifically written or available in sheet music for a Bb trumpet and that is where my ability to sight read piano, voice, and guitar (PVG) music has been a blessing to me given by Mother playing piano and my band director because we had to if we wanted any music at al. during the scarcity of WWII and we scrounged to borrow such from the whole community and hand copied our parts, transposing as we went for whatever instrument we played.
As a very young boy before puberty, we were taught in grade school the lines of spaces in the treble clef with a word or ditty: for the spaces it was the word FACE, and for the lines it was the ditty Every Good Boy Deserves Fudge, but later in my life I put these in sequence as EFGABCDEF. Still, later in band was taught the space below the staff was a D and next on the line was a C. For a moment let me stop here, to tell you this in the only line between the treble and bass clef or as some say the G clef and the F clef. A pianist calls this a "middle C" as it is just left of center on an 88 key piano keyboard. Should you descend further and alternating space and line, next will be a B in the space and the next will be an A on the top line of the bass clef and you can now fill in the blank ____ with the next letter. Did you notice there are only 7 letters ... it's easier than the English alphabet. Going back to the grade school pattern and alternating space and line we can ascend above the staff.
However, all this for piano pitched in the concert key of C, and you are about to play a Bb (the "b" representing a flat for this computereeze). Visualize in your mind that piano keyboard as is a mixture of white and black keys. All the white keys are just a repetitive pattern of the letters I've just mentioned. If there is one, a black key to the left of a white key is a flat of the white key, a "b". Now if there is a black key to the right of a white key it is a sharp of the white key to its left symbolized by #. Whether to the right or left of a white key the black key is a semi-tone apart from the white key. Where there is no black between a pair of white keys, these white keys are also a semi-tone apart.
Now you're ready to play your Bb instrument! Remembering what I just stated, you'll realize the Bb is 2 semi-tones lower than a C, but when you actually want to play that piano music and sound a C you see before you in PVG music on your Bb trumpet, you must play 2 semi-tones higher and this process is called transposing.
Is one of your harmonicas a C chromatic? If so, you may understand, and playing by ear you can duplicate the tones of a piano.
END OF MY LESSON.