Change default database of a login:
alter login <loginname> with default_database = <dbname>;
Create a user in a database for a given login:
use <dbname>;
create user <username> from login <loginname>;
Make a user member of db_owner group:
use <dbname>
exec sp_addrolemember 'db_owner', '<username>';
Make a login ‘dbo’ of a database:
alter authorization on database::<dbname> to <loginname>;