some people say if u don't know how to overclock, don't do it. but then, how will anyone start? overclocking is relatively safe, in the way that i reckon u gotta actually try to cause permanent damage to ur computer
gpu:
overclocking gpu is easy - cuz ur computer doesn't come up with a blue screen of death the second u overclock too much. there are software tools which overclocks ur gpu automatically and finds the point at which artifacts begin. it's always better to do it manually tho, but software tools do it admirably
cpu:
cpu overclocking has to be done manually thru the bios. some motherboards don't have overclocking features in the bios, especially oem ones. commercial (enthusiast) mobos have a ton of features. there's no quick guide in 1 post so u can go off and do it, but i'll try to give an overview:
the trickiest thing about overclocking the cpu is that the cpu is sync'd with ur memory. so if u overclock your cpu, ur also overclocking your memory. nowadays, most cpu's have a lot of headroom to overclock, but most memory (generic brand ram) has hardly any overclocking potential. so an overclock on an overclockable cpu will fail cuz the memory is holding it back.
the cpu clock speed is the fsb (front side bus) * multiplier. fsb is 200mhz by default, so if u have 200fsb*10 multiplier, u have a 2ghz cpu. multipliers are locked, so overclocking is done by increasing fsb.
memory is also 200mhz default, and increasing fsb also increases memory speed. lets not get into memory overclocking, because that's a whole different beast - much harder than cpu overclocking, and ur ram's probably generic. u want to keep your ram at 200mhz or lower while increasing fsb, and that's done by running a divider. that means if ur running a 4:5 divider, ur memory will be running at roughly 4/5 ur cpu speed (but it's not simple arithmetic). also, there's only a few dividers available (4:5, 2:3 - too few)
to overclock, u want to keep bumping up cpu fsb speed, while keeping ur memory as close as possible to 200mhz. then u boot into windows and run stability tests. stop at the first sign of instability.
cpu overclocking is easy once u know the theory, but the theory is a pain to learn. the upside is that nowadays, cpu's can be overclocked +1ghz on air, and that makes it worth learning