Welcome in Arduinoland,
my first advice is to go through the tutorial section (for several days) to get better understanding of how the programming language of Arduino works. (as I don't know your prog skills) .
A calculator needs to read a string (lets say from the serial monitor of the IDE) and parse that, Parsing is the splitting of a stream into meaningful parts. e.g. The stream
12 / 13 =
needs to be split in
Operand1 = 12
operator = divide
operand 2 = 13
Note the = sign can be used to start the parsing.
If you have those three parts you can (based upon the operand call the function
float divide(float a, float b)
{
if (b == 0) ... code to handle the error ;
return a / b;
}
which returns the result.
Hopes this gets you started.