You may want to consider starting over. Harsh, I know, but sometimes it's the only way. Here's a link that can help.
http://www.allaboutlawns.com/backyard/reclaiming-your-lawn-how-to-replace-a-lawn.php
Otherwise, yes, they say you need a thick lawn to crowd out weeds. I'm still trying in mine and it hasn't happened yet. Keep pulling the weeds and do spot weed killer as needed.