Moving from sub account to new main account

Hi - my brother has a main account here, and he’s been hosting my account under it. I’m now ready to move on and get my own main account. How do I do this? When I try to sign up for an account and it asks me for my domain, I type it in and then I get a message that says that it’s already in the system. I need to start my own main account, where I will be hosting a few domains myself. Someone please help!! Does my brother have to delete my sub account before I can assign it as my own new main account? I’m lost!! PLUS, I don’t want to experience any down time, or at least, very minimal down time, and set up everything again, etc. Thanks!