Screenwriter, producer, teacher, novelist and bon vivant William Rabkin has written an excellent article on Storylink about what a newbie writer has to do to break into television. You've still got to write an episodic spec script, but…
Your spec can’t simply be a good episode. It’s got to be bold, audacious, and big. It has to go places no one has ever thought of going before and do things no one has imagined doing. And it’s got to do it on the first page. Hell, on the first half page, because your reader may not bother going further than that. You’ve got to grab your readers right away and force them to keep reading.
In short, you need a gimmick.
No, typing your script in 3-D and including polarized glasses isn’t going to do it. What I mean by a gimmick is a transformative approach to storytelling that allows you to retell the series’ underlying narrative in a way that makes it seem new again. It’s a stylistic or structural element that shows that your vision is so intriguingly different that showrunners will fight to bring it to their series.