If you are new to CSS Form Builder you should start out by checking on the support page, here is a link:
In here you should start with the getting started guide. There is an option for a login type of form in the form builder presets so you can use this for a login form. This will only create the form though, to make it work with your db and users table you will need to make use of Security Assist. If you use this tool on the login form you can link it with your users table.