The overclocking options offered by AMD OverDrive are broken into two broad areas: Novice Mode and Advanced Mode. You can swap between modes at the top of the Preference tab.
Novice mode allows you to use a single slider to control overclocking of your system, selecting between levels 1-10. You can't achieve very much overclocking using this mode, but it is definitely the safest way to go. It overclocks every component by a small amount, which you can see as you slide through the options.
Also available in Novice Mode are the Benchmarking tool, which tests the CPU, Memory, and Cache, and the CPU stability tester.
Advanced Mode unlocks a ton more features and options, but also doesn't protect you from making big mistakes that will destroy your system. I recommend you don't even look at advanced mode unless you know precisely what you're doing. Advanced mode gives you full control over clock speeds and voltages on all possible components. There is a relatively simple way to overclock just your CPU using a percentage selector, but you need to be cautious, not just crank it up to the highest possible and "see what happens".
Advanced Mode also opens up a whole new tab dealing with memory, where you can set timings and voltages as needed - again, only do this if you know exactly what you're doing.
Auto Clock Utility
The Auto Clock Utility is designed to test the limits of your hardware configuration's overclocking potential. It is worth noting though, that this process will often freeze your computer to the point that you need to do a hard reboot. This happens because the program pushes your configuration to a limit that it cannot support, causing it to halt. Once you reboot, go back to the Auto Clock area and it will tell you what your maximum successful overclock achievement was. Input those numbers in advanced mode, and run a stability test to see if you can work with them.