Motherboard wise it really doesn't matter, AS Rock offers alot of bang for the buck and good quality, same with MSI. ASUS is still considered the best and I've heard good things about Gigabyte. I would get a B450 board, mATX or ATX is up to your case.
For RAM Ryzen 2600 is less picky but you honestly cant go wrong with Corsair Vengeance LPX and GSkill Ripjaw is also good (and usually cheaper). I would check the motherboard manufacturers support page for each board and see the QVL list for RAM compatibility. That's not saying a RAM brand and model that isnt on it wont work but it's still a good reference. Whatever you choose buy the ram in a matched kit, so for 8GB get the 2x4GB or for 16GB 2x8GB. Youll want duel channel memory...