How do I convert my website in a multisite

Hello, I am a new subscriber, your advertisement said that I could convert my own website into a mutisite market place. So far it's not working for me. the framework screen does not appear.
Can you hep me find a way to build my multisite?

The main reason that I subscribe for 1 year is that I intend to build many websites for my customers by down loading and installing themes and plugins. would that be a problem with my subscription?