SirSky :
I want to know what are all the parts in building PC?
And What are the functions of each part?
How to know how one part is better then another (1 cpu to another cpu)how tell be able to tell which is better?
so if i buy all parts they will just fit in?
every parts will be able to go in with another?
will i have to buy specific case, monitor etc for certain parts to be able to function.
Since its your first build I bet you are on a tight budget,yes? If so go look at one of the AMD kits on Tigerdirect, where you can get everything you need at once and save some $$$ in the process. Your best bet ATM would be the FX4130 kit they have right now, it gives you the whole smash, quad core CPU, 4GB of RAM, a 500GB HDD, DVD burner, along with a case and power supply for $260 after rebate. Add a copy of Windows and an HD7770 and for around $430 you'll have a PC that will play most of the games out there at an acceptable framerate.
As for the parts you'll need just look at the kit. You have the CPU, the heatsink for the CPU, the memory, the motherboard, the hard drive, the burner (makes it easier to install the OS), the case and the power supply. As far as "which is better" that is impossible to tell without knowing which software you plan to run but if you are playing games pretty much any of the quads by either company will do and faster speed is always better on that front. With a kit you'll KNOW it'll all fit, that is why they sell it together after all. As far as monitors go no, pretty much any modern monitor will have both VGA and DVI and since the boards come with both in most cases you'll be just fine.