The fast crawl option is greyed out in my webmaster page on google. My site is quite new so what would it take for me to be able to use this option?
I'm not sure what it will take for you to use that option. However, if you build enough backlinks to your site google will start crawling your site more often.
I think what you need to have google assume you could use the fast crawl is LOT'S of new content and a fair number of good quality inbound links. If they think your site is important enough (judged by incoming links) and they're not keeping up with your content "production" at their current crawl rate, then they'll ungrey the option for the faster crawl. From what I've seen it's entirely automated.
If your site update often or you add new content very often, then you will see this option available after some time.