Blimey....that's a good starter question.
Will get back to that when I have more time.....If nobody else fields it first.

But basically, to overclock is to run your processor/ram/graphics at faster speeds than they are "officially" rated to run at.

It can be a tricky and time consuming process.......but is fun and can be rewarding.
But it must be approached with care.

Anyway....more later

ps.......perhaps you would like to repost your question in the hardware thread?

I would guess that the best place to start is with a bit of reading. I don't like giving RTFM type answers but in this case, have a read and then come back and ask questions if required.

Here is a good page