abandon all hope and do not even think about starting working on a cpu without the full register documentation (which is often not available without an NDA)
pick one of the three big ones (raspberrypi, arduino, esp*)
pick one of the three big ones (raspberrypi, arduino, esp*)
My suggestion is think up or choose a project you think might be fun, then work out (or ask for advice) on which of those three choices is most suitable, and go learn how to build it.