How to choose one

Currently, I am a coach and want to build my business in coaching for Japanese people soon. (I am Japanese).
However, there are many things I think I should learn to make my own business such as marketing and building up my skill.

I think I should learn and make sure about my niche, my purpose, my vision, my strength etc.. but at the same time, I also feel like I should build up my skill first.
In a video, coach mentions to choose one to focus on.
However, I am not sure how to choose and prioritize one thing over another.
That would be great if I could have some ideas how to do that.