how

[Julus]

If I buy a new motherboard, how can I know which graphic card and processor to buy?

[sttubs]

Motherboards have a specific CPU socket ie: AMD has AM3 and AM4. Intel has many different socket numbers. All modern graphic cards will work in any motherboard. You will also need to get the right memory. You have to first answer what you want to do with your computer ie: gaming, rendering, or just web browsing. Then research your parts and come up with a budget. PCper has put together the leaderboard that has 4 different types of systems and budgets: https://www.pcper.com/hwlb
Feel free to post any other questions you have.

[Megaman_X]

They should have list of specifications that will list all the information you need. for example, Newegg will tell you the socket type, memory capacity, memory standard, form factor, pci slots, and everything else that should be fit on it.