Use an outliner

I'd say pick one of Workflowy or Roam
I'm biased towards Roam because
- I already use it
- It works for me
- It has a lot of "power features" that go beyond jotting down thoughts
But ... Workflowy is "good enough", especially to get started somewhere with something, which is a whole lot better than what I did for a decade before, which is ... nothing
Some nit-picking reasons I prefer Roam
- Each "bullet" has its own timestamp and version: I use this to figure out things like "time taken"
- Text can be formatted to have headers or separators: I find this useful
Pick one of them and start using them. If you're totally new to this, I'd recommend Workflow to start with.
(Yes, I wrote this in outline form and am sharing it as such ...)