Nice work! Here's some tips that might help you out:
INPUT only reads a number as input. To get a string like "+", you should use LINPUT instead. However, just changing that won't work, because string variable names have to end in $. Therefore, that line should become:
LINPUT "OPERATION (+-*/)";B$
and B should be changed to B$ elsewhere too.
It's a good idea to avoid GOTO. It's almost never necessary and tends to make programs confusing or lead to bugs.
For instance, this works:
IF B$=="+" THEN D=A+C
IF B$=="-" THEN D=A-C
etc.
whereas this has a bug in it:
IF B$=="+" THEN GOTO @PLUS
IF B$=="-" THEN GOTO @MINUS
@PLUS D=A+C
@MINUS D=A-C
Notice that after D=A+C is run, it ends up running D=A-C too.
This program starts with BGMSTOP and CLS to stop the music and clear the screen, but there's a more thorough way to do this that might be helpful.
SNDSTOP stops all currently playing sounds, whereas BGMSTOP only stops music.
ACLS resets the whole screen, whereas CLS only clears text, not graphics.
If you would like to get into the advanced part of calculator building, that uses order of operations to evaluate expressions, look up Reverse Polish Notation, which is done using the Shunting-yard Algorithm.
