Wow! A deceptively LARGE question!
For me the first step is for YOU to understand why you want to do this. And not to stop at your first answer – why, why, and why?
And then what does success look like – if the business “succeeds”, what will it look like, what will you be doing, what will you be or have that you aren’t or don’t now?
Then next thing is to be really clear about what problem you business will solve, for whom (ie. what type of customers are your really core sweetspot).
Write all your answers down, and keep them so you can remind yourself later, when the going gets tough (and it will) what your purpose and plans really were in the beginning.
The actual mechanics of “starting” are relatively easy. It’s understanding why you’re doing it, and what your hopes and dreams for it are, that is really important.
Good luck on taking your first step / giant leap!