Understand what makes up a computer and why each part is necessary. Then try to understand what kind of performance hit you will get if u skimp out on stuff. Then research about what are the best "bang for the buck" stuff they have in the market for each type of component. The techist.com is a great source for that.
You have to understand that not everyone can afford the best of everything. You have to make compromises. And the trick is to put ur money where it will give u the greatest outcome. Buying the best RAM to overclock is great but if u buy a slow processor, then what? Maybe buying RAM that is not "so" great for overclocking (but good) but getting a more decent processor might give a better performance overall.
After that, if u are still not sure of stuff, u can post more messages