First buy all the computer components and put together which is the first part of assembling.Assembling must be done carefully to avoid unnecessary damage to the system.Materials required for assembling are mentioned below. All the necessary components(Although the all the below components are preferable, not all are necessary. Then necessary ones are marked with a *)
Processors *
Motherboard *
Hardisk *
RAM *
Cabinet *
Floppy Drive *
CD Drive *
Cards
Display Card (Not needed if On-board display is available on Motherboard)
Sound Card (Not needed if On-board sound is available on Motherboard)
Modem
Other Cards(If Any)
Monitors *
Keyboard *
Mouse *
Speakers
UPS
Other Components(If Any)
Also keep the cables that came with the components close by
Philips head Screwdriver (Or Star Screwdriver)
Flat head Screwdriver
Forceps (for pulling out jumpers and screws)
Magnatized Screwdriver
Multi meter (Testing)

Environment Required
Make sure that the following things are true of the room in which the computer is assambled/kept.

Make sure that a flat surface of a good area is available when the system is assembled. Make sure that the room has enough space to move.

See that the place where the Computer is kept is dust free as dust can harm the system.

Make sure that the room has good ventilation and won't be very hot.

Check the grounding in the plug to make sure that earthing is done properly.