Infix, Postfix, Pseudo-Calculator using Stack ADT

basically, what i want to do is create a generic pseudo-calculator which takes a string containing a mathematical expression in standard or infix form, converts it to postfix form, convert the characters to respective int values, then perform the calculation, and finally output the result..

i already have the code which converts infix to postfix using stack ADT, and the code to "calculate" the postfix using stack also...

can anyone give me a pseudocode for the main function?? my main problem is converting the characters in the string to their respective mathematical tokens.

thanks!!!