First you decide what you will use the PC for. If you are going to be playing modern 3D games you will need a good graphics card. If you are going to use a lot of applications you will require extra memory. If you are going to run mathematical or 3D rendering or video editing software you will need a powerful processor (CPU). When selecting the software you want to use make sure you build your pc with the hardware that is required by that software (the hardware requirements). When you know what you want decide which CPU you want and what graphics card you want and the amount of memory. Then select a motherboard that is compatible with the CPU and graphics card you choose and that it supports the amount of memory that you want. There are many motherboard so take your time selecting it. Make sure you select one with network (maybe also wi fi if you need it) and audio support. The motherboard is the main component of you system so make sure you do not select one of a cheaper brand. Check if the motherboard of your choice supports everything you need (USB, Firewire) Then select a DVD ROM or, if you need it, a DVD writer. If you require to play or write other optical discs (Blu Ray for instance) then choose the appropriate hardware for it. Choosing a harddisk is easier. Basically you choose based on the amount of GB. Ofcourse you need a case where you can put it in. There are different sizes of motherboards (ATX, mini ATX, BTX). Make sure the case supports your motherboard. Some cases have a power supply built in. If not, select a separate power supply. Make sure the wattage is sufficient for your system. You can view the specs of the hardware for information about this. On the ASUS site is a calculator to calculate the amount of Watt that you require. Select a case of which you like the design. Take into account if you want to use it in your living room as a media center for instance. Select a keyboard and mouse that suites you. If you want to do gaming then select a mouse with extra buttons and a high precision. These should be USB devices and your motherboard should support USB (the current standard). The monitor you pick by the size mostly. There are however monitors that support 200 Hz which can be used with modern nVidia cards to combine with 3D glasses that give true 3D vision in full color. If you choose a DVI monitor then your video card must support it. Maybe you also need extra peripherals like a scanner or a printer then just choose the ones you want. If these are USB devices which you motherboard supports then you can easily connect them.