You have the Z68 Extreme3?
Start by installing
CPUz and
Prime95. Run Prime95 stress test (small FFT first) and take note of CPU voltage on CPUz.
Then, go into BIOS (hit delete key before the windows logo shows up on boot) and set all-core CPU multiplier to 42 to give you 4.2GHz for starters.
What CPU cooler do you have? Not the stock Intel one I hope?
This chip is old and has been OCd by thousands of people. A simple Google search can give you tons of OC guides.