As mentioned, you have two options only: either get an AMD APU, that is, the Ryzen 3 2200G(which is 4-core 4-thread) or Ryzen 5 2400G(4-core 8-thread), or you need a dedicated video card - even a cheap one is fine if you won't play games. The other Ryzen chips don't come with integrated graphics(nor do the motherboards - that happened a long time ago), so they need a discrete GPU to be able to run a monitor.
Edit: Third option, of course, is to get an Intel chip with a supported motherboard, in case you don't already have an AM4 motherboard.
Edit 2: You also don't really need an X370 board, unless you plan on extreme overclocking or SLI/CrossFire(which seems highly unlikely since you don't even want to buy one GPU, forget buying two)...