Database Setup with Windows Installer

I've got an application that requires a SQL Server database. I have added a custom action to my setup project to create the database during the install. That part works fine. What I need to do now is set up a user for the database. That's what I'm not sure how to do. During development, the app has simply logged in as the 'sa' user. I'm definitely not a SQL Server expert, so I'm not really sure what needs to be done to create a user that the application can login as. From what I can tell, it looks like I need to have a database user set up, and that user needs to be tied to a Windows account. What info do I need to prompt the user for during installation, and how do I use that info to set up a user for the database that my application can use to login?