Well, remember that during normal, everyday use, you might be using a few percent of your computers capability. Unless you're doing video rendering or what not, if you hit more than 10% load, I'd be surprised. Right now I'm watching some youtube, have a few tabs open, and am typing this response out. Load is about 11% (laptop, i5-2467). A full blown desktop has much more power.
So - say your overclocking increases your capability by 10%. That means instead of using 10% of your computer, you're using 9% or so. Thus you get a 1% reduction in CPU usage.
On the other hand, if you're running a game which is CPU limited (let say your CPU is running 99%). If you overclock and get a 10% increase in performance, your computer thus can utilize that 10% increase (if the CPU needs more oomph), or if it's -almost- CPU limited (99% usage), it would drop to 89-90% total CPU usage. You'll notice a 10% increase.
You just can't notice a 10% overclock when the computer is only loaded to 10%, it's when it's really loaded that you'll notice a 10% overclock.
As for how to overclock, there are quite a few things to have first.
1) Better CPU cooler. Overclocking generates more heat, that heat needs to be removed, and usually the stock cooler is marginal.
2) RTFM (read the freaking manual)
That will explain where the settings are.
3) Google your motherboard model and CPU and add in 'overclock'. This would probably pull up what other people have achieved with your mobo and CPU combination.
Be aware that not all CPU/mobo combinations can overclock. Cheaper boards especially. Most Intel non-K CPUs with a good motherboard will only get maybe 2-5% OC by increasing the BCLK frequency, and if it's non-K, you won't be able to adjust the multiplier.
If it's an AMD setup, you again might only be able to get a few percent, and you'd have to again adjust the BCLK.
Overall, you're best off getting a -k Intel setup and going to town with a good cooler if you really want to OC.