If you try to finish your registration and get the error message "Login already in use", this means that someone else was quicker than you and has already registered an account with that very same username. The solution is simple: Just use a different username.
Since we have more than 3 Million registered accounts, all the very simple usernames may have already been taken. If your second and third pick appear to be already taken as well, try something a bit more complex, like adding your year of birth to the name.
Just make sure that the user name matches the required login format. If it doesn't, we recommend taking a look at this article: Error during registration "Login format invalid".