How do I find out who my target users are? [closed]

I'm currently reading "just enough research" by Erika Hall and she seems to completely skip this step. Ideally you want to recruit people to conduct user research on who share the same traits as your target audience / users. From there you can analyze data , create personas and then come up with design solutions. However I'm lost on the Initial part of discovery, which is finding out who my target users are. Can anyone shed some light on this?