Your processor, RAM and video card all run at a certain speed. Usually somewhere between 3GHz and 4GHz for processors with RAM varying by a very large amount depending on what generation it is and even it's speed (DDR2 is slower than DDR3 and DDR3 1333 is slower than DDR3 1600). The speed of the RAM and processor is determined by the motherboard when the computer starts. It looks at the processor and checks to see what speed it should be run at, same goes for RAM. Graphics cards are usually governed by hardware on the card itself. It's also worth noting that some cards come factory overclocked (a very conservative overclock in most cases) but can also be pushed a little bit harder.
With overclocking, you are pushing these clock speeds of these parts past what they are designed to normally operate at. This is done in an attempt to get better performance (usually does yield some sort of boost but the amount varies between applications), but there is also a cost to all of this. Overclocking a processor immediately voids its warranty, it also produces more heat so the cooler that comes with it probably won't be enough. It uses more energy to run the processor and shortens its lifespan.
I wouldn't even bother overclocking the RAM, it's incredibly difficult to do and often offers little to no noticeable performance gain.
Overclocking anything on a computer takes time and patience. You have to set a clock, start the computer and run a stress test (for several hours) to see if it crashes. If it doesn't crash then you have a good overclock. If it does crash, you have to restart and either dial the clock back or add more voltage or both. Even after you get it stable, the odds of a crash are still there and are higher than normal speeds. Everyone will get a different clock speed that is stable, even if the same processor is moved to a different motherboard.
For gaming, overclocking the processor and RAM offers almost no performance gain. There is a video somewhere of a tech group that was testing this and they only saw a minor performance game in tripple A titles, I'll see if I can find it. Overclocking the graphics card usually does yield a better frame rate though and depending on the game might be worth looking into (overclocking graphics cards is also quite a lot easier than overclocking processors).