You need to contact them - that's the basic, however if you contact them they may or may not cover you. Now to get a coverage you have to pitch to them well. You can follow the links below for some insights into it:
http://www.mediadeluge.com/post/32818616/how-to-get-techcrunched-part-1 http://robertomartinez.info/posts/how_i_got_techcrunched.htm 2 The rest is luck. I have one more suggestion why do not you try HackerNews and ReadWriteWeb:
This Quora question has answers from a few people who work for TechCrunch answering this question: http://www.quora.com/TechCrunch/How-do-you-get-TechCrunch-to-cover-your-startup Basically the answers all say the same thing including the ones from staff who work there like MG: they get hundreds and thousands of emails per day with pitches so it's luck whether or not you're covered on TechCrunch.
My personal advice is to create buzz elsewhere on other sites and news websites, on Twitter, etc and then TechCrunch will write an article on your start-up without you having to pitch to them. Don't focus all of their attention on the one outlet, TechCrunch is one of many places that can have a viral effect on your start-up being covered.