The short answer: Get a customer.
Of course there is more to it than that. Usually there is the decision to be in business. Then preparation. Then selling the first customer.
This is not a linear process. Some may be on the step of making a decision until they die without ever starting anything. Or some who have definitely made up their mind spend their life getting ready.
And some start with seemingly no decision and no preparation. But if you get them alone and listen carefully you can here some moment when a decision is made, often with the preparation already in place.
A very, very successful business person told me this moment happened on a ski lift, the preparation had already taken place with education and job experience he suddenly saw. Until that moment he'd never thought of being in business for himself. Getting fired or treated poorly in some other way by an employer often has this effect.
The best book for people who are starting? Just Start, Harvard Business Review Press. Go to Amazon, search on Just Start. While you are there take a look at my little book, very simple, talks about the four phases of startup, search for it on Amazon with my name John Wren Daring Mighty Things.
This life, this day, is very short. Let's go get started!