I suggest you go this website PCPARTPICKER that many Users use to build their own computers: https://pcpartpicker.com/list/
First decide which Motherboard you are interested in then see what APU/CPU and RAM are compatible with that Motherboard. The Website will show you which are compatible.
I suggest you try to purchase a 500 series Motherboard so at least it will support the latest AM4 AMD processors. But the 400 Series which may be slightly cheaper some may also support the latest AM4 processors.
The RAM and CPU will probably cost you the most.
Personally I tend to be biased towards Asus Motherboards. I have had very good experiences with them.