When do you actually start your fitness journey?

You "start" when you are all in.
It's all part of the process, it's all part of the journey.
Doing what you can, doing what you are comfortable with, doing what you think you should is ok,
But you can't be 1/2 pregnant.

Until you are doing everything, how are you supposed to know if it's working or not?

I've said it several times, the 100% required vs the 100% you are willing to give are not always the same.
But the 100% required is what is needed to see if what you are doing is working or not working.