If you are getting an error message or the script just doesn't work at all, this is the place to be. If you just have a question about something, go to the FAQs
Warning: main(WHATEVER/functions.php): failed to open stream: No such file or directory in WHATEVER/index.php on line 21
This means you set the paths wrong. If you're not sure, ask your host. Then when you know the correct paths, you'll have to manually set $CONFIG['path'] and $CONFIG['templates_path'] in config.php. After you're done, upload config.php and the error should go away.