A very good question! The best way to start an online business is by creating a website and having a blog where you will write about your products or services. This gives the potential customers to know more about you and if they like what they read, then they can buy from you.
The second thing that I would suggest is having an email list because this will help you get more clients and it also helps your site gain more traffic.
Another important thing is having a marketing plan because without one, people won't know who you are and how to contact you.
So, I believe this answers your question! Thanks for reading!