Many FTP programs have a setting for the initial remote directory. When you FTP to your site without setting this option, you will be in your /home directory (which is not accessible on the web) and you will need to navigate to you www directory to upload web-accessible files.

If you want to FTP directly to your web-accessible directory or if you are using a web development application that moves the files to your web directory automatically through FTP, you will need to set the initial remote directory to www/