A big help is typing MML and looking at the help. It gives you a lot of information on what can be included in an MML string, which is what the program uses to play music. Use BGMSET to save music to play later with BGMPLAY.
Letters A - G will play those notes. You can use # for sharps and - for flats. Writing a number after a note designates the length of the note. You can tie note lengths together with &.
To use an instrument other than a piano, use @ then a number. The built-in Smile tool is useful for hearing what different instruments sound like. If you want multiple instruments playing together, you can designate channels for them to play on with : (colon).
More about notes...If you want to change the octave notes are playing, you can designate with O (capital o) and a number, or use < to go up an octave and > to go down an octave. V and a number can control how loud the music plays, or ( (open parentheses) to increase and ) (close paranthese to decrease volume.
You can change the tempo with T and a number. If many of your notes have the same length, you can instead write L and the length value to set a default length. That means that if you don't write a number after a note, the program will play the note for the length of the value after L.
