THE MOTHERBOARD

The Motherboard is like the skeleton and the central nervous system of the computer. It is the basement of everything else in the computer and if the motherboard fails, you have to get a new one and basically rebuild the computer. Motherboards too range from being cheap to expensive, but you don't need anything more than a mid tier motherboard unless you're doing overclocking or have a high end CPU.

ITX, M-ATX, Or atx?

These are the sizes that exist and they go from small to big, they can go from about $50 to up to $250 but this will depend on what features you want, some come with Wi-Fi included, some with better internet or double LAN ports.

Check the combability with CPU

Some motherboards will only will work a certain type of CPU socket so you must check first what socket your CPU will need. AMD and Intel also use different sockets too so you have to make sure you're getting the one for your system.