<edit>You don't say if you want to issue user names and passwords personally - rather than by members registering through the site. If that's so, and </edit> if it's not obvious from your domain host that they accept restricted directories, google for htaccess and htpasswd.
Don't forget to check with your domain host that they support htaccess before you go down that route, though.
As for forms, search this forum - and the web. Same goes for passwords and registration through the site - which is very much server-side stuff.