Your basic pc is made up of a case, Central Processing unit (cpu), Hard drive, Memory (ram), Motherboard, Graphics card, Power supply, CPU cooler, DVD R/W and operating system. Basically it's a case of researching what parts work with each other, determined by the motherboard, and putting them together.
The parts I bought are all high end, so it will last a good few years. If I was to buy the same spec pc I'd probably be looking at over 3k.