Whether you build a new PC or upgrade the existing one, the motherboard is the most vital component to choose when you have already picked the processor of your choice.
 

It is important to keep a few things in mind while purchasing the mainboard: 

Pick up the Right Socket: You can single out a great CPU from either Intel or AMD, but whatever CPU you choose, make sure your mainboard has the right socket for the selected CPU. The latest mainstream AMD chips use AM4 sockets while current Intel 9th Gen Core CPUs require LGA 1151v2 sockets.

Motherboard Sizes: Motherboards typically come in three sizes from largest to smallest: ATX, Micro-ATX, and Mini-ATX motherboards. ATX motherboards offer the highest number of RAM and expansion slots. If you are looking for a budget build, you can use a smaller chassis with the micro or mini boards, but you'll have to settle for fewer PCIe slots, RAM banks, and other connectors.
 

Number of RAM Slots: Most mainstream boards these days have four RAM slots, although Mini-ITX models often have just two, and high-end boards frequently offer eight. The amount of slots, of course, limits the amount of RAM you can install.
 

Motherboard ports: It’s essential to check the I/O area on a motherboard to make sure it has the external connection options you need. You may need more USB 3 ports while others may want more audio ports. Among the most common ports, there are USB 3, USB 2, audio, USB type C, USB 3.1 Gen1 and Gen2, HDMI, Display port, PS/2 and Thunderbolt 3 ports. Ports one probably want the most are USB 2 and USB 3 Gen1 ports, as the first one is perfect for keyboard, mouse, and many other devices and the later one work with most peripherals. USB 3.1 Gen2 and USB Type-C ports will come handy in the near future, although most peripherals do not support this standard yet.