The next step is to find the derivative of this, to get the second derivative. Before starting to take the derivative, I'm going to simplify a little bit, so it's easier to work with. Let me know if you need more examples. Between these points the function is either increasing or decreasing. If the function has no infinities, it's fairly easy to see what has to happen.
Thinks can be a little more complicated if the function has some points where it is infinite.
For more hints, see this page. If you have further questions, email me.