Can someone explain me step by step how to set up a hosting business? Thanks in advance! :)

There is more than one way to do it.

You can become a reseller for a large hosting company, in which case you only need to sign up with the provider for whom you will be reselling. They will handle everything; all you need to do is to go get clients.

Alternatively, you can go your own way. Lease or co-locate a dedicated server or a server cluster (in the extreme, build your own data center), develop or procure account management applications, hire sales people, system engineers and support technicians, and all that jazz...