restricting access to folders on server

i am using apache server and presently when i try accessing any folders of my website i am able to browse the files ex = www.website.com/images which is a serious security risk as i am building a forum website using php and mysql. in the root directory i have created a .htaccess file and whenever someone access a file which is not on the server i have created a user friendly message that the file does not exist instead of a 404 error message displayed by the browser. similar to this how can i go about restricting users to browse all my folders in the toot directory. if anyone accesses for ex = www.website.com/phpscripts an alert should appear asking them to enter a username and password. 1. how can i do this using apache. 2. where do i write the username and password information and will this apply to all the folders in the root directory or specific directories. please advice. thanks.