creating new users with dbo permissions

I am trying to create a new user for a SQL Server database and use the credentials in an
ASP.NET app.

Problem is dbo permissions are not being applied to the database objects when I set up a
new user (Logins -> New Logins) with SQL Server Authentication, set the default database to
the database I want.

I then get an error message saying that the user has not been granted access to their default
database : DBNAME and therefore will not be able to gain access to their default database.

I'm not sure why this is as I am logged in as sa.

Anyway, when I ignore the error and set-up this new user through the Users section of the
database none of the dbo permissions carry over.

Can anyone help?
Cheers.SQL Server's default authentication is via Windows only and you have to invoke
authentication via SQL Server usernames/passwords by right clicking on the server name, selecting the Security tab and making the change.