Ok so I would recommend reading
THIS article. It really helped me. Also, I wouldn't recommend just immediately upping your voltage before even raising the multiplier. Raise the multiplier slowly and after every time your PC boots use a test like Prime95 to make sure the OC is stable. Then once your PC isn't stable raise your voltage by .025. I am running @4Ghz with a voltage of 1.375 because I don't want to risk raising the voltage past that. But don't just copy my specs to your build, you could destroy your CPU that way. You have to remember that not every 8120 will reach the same overclock, and our cooling solutions may vary.