Hi friends!
I would like to build a new PC..
Actually I don't have any experience on building a PC.
So please help me on what types of hardwares I have to purchase?
What
Processor, ram, hard disk, I have to choose?
And also tell me what other things I have to consider?

Why do you want to build your own? What will you be using it for? Do you have a budget?

I just want to learn the things.
Actually I need the source, So that I should not make any mistakes while building it.
Budget is the second thing.. I already have monitor, keyboard and mouse.. Now the only thing is I have to build a CPU for normal everyday use.. Actually I'm looking for the following config

Dual core pro
4 gb ram
320gb hard disk
Original intel m/b
Writer

And also let me know what other things have to be consider..

You please guide me to build it.

One more thing, I would like to present a laptop for my sister.. can you suggest me one.. my budget is around 600 US$

You don't build a CPU, you buy a CPU to place on a motherboard. Building a PC is simply a matter of choosing the components you want and assembling them.

For the specs you list, you would be better off buying a stock model. You will save money and make sure everything works.

As for the laptop, any Core i3 machine with 4 Gs of RAM will suit and be within your budget.

First what are you going to use the system for, Surfing, Gaming, school work. Do you plan to run Windows and if so what version? Did you want to run Linux or make a Hackintosh. Once you decide what you are using the computer for you can get some idea of what you will need. Remember to double all minim system requirements for Operating systems as a starting point and build from their. Say Win 7 home needs 1gb of RAM then you start at 2 GB and if your applications need a lot of ram you should increase it. A good starting point is 4GB. Always build as big as you can afford. You can save money by building into an existing case but get a new power supply.