There are many algorithms to convert infix to postfix all over the web. But my question is how to make that to support functions? For example sin(x+y)*z.
The code you'll have to work out yourself. Using your specific case as an example might help get you started; the postfix form of sin(x + y) * z would be:
x y + sin z *
Note that in this one example some operations operation on two values (+ and *), and others one (sin)