You need a system case, motherboard w/ a processor that's compatible, hard drive, RAM sticks, a power supply that will be able to give enough power to everything, an operating system (Windows, Linux, etc.), and a monitor. Then you need to make sure you have all of the extras that you'll use, such as an optical drive, mouse, keyboard, etc. Not to mention you need the right cables for everything such as ATA (ribbon cables) or SATA cables.