Finding a quadratic function the bounds any function below.

Question

Given a function $f(x)$ I need to find another function $g(x)$, where $g(x) = Ax^2 + C$, where $A$ and $C$ are constants, such that $g(x) \le f(x)$, for all $x$. I can use a computer to do this for me but:

1. I don't know how, and
2. I want to know the process.

Thanks!