James Shore's Lets Play TDD (http://www.jamesshore.com/Blog/Lets-Play) is an excellent video series that has the perfect pace and level of depth for a beginner. James Shore is also doing a Javascript series (it was a kickstart project, http://www.letscodejavascript.com/)

Specifically regarding your question of knowing where to draw the line, there is never going to be definitive answer. That is one of those questions that changes frequently based on your specific project and codebase, but overtime you get a sense of where those lines generally are.

My advice would be to watch the first few videos, then start trying to do TDD on a small project.