With the data you gathered from the analytics tools, you can define a strategy towards what you want to build. This is particularly useful if you already have a desktop version of the website.
Google Analytics can help in finding out the most commonly used mobile devices accessing your site, and also find out the most popular sections of your website.
You can also determine what are the most useful pages on your mobile site. People treat mobile surfing differently from desktop surfing. For instance, if you are running a local store selling products, most people on a tend to surf pages like Contact Us, Location, and Services mobile device. Conversely, on a desktop, people tend to search for Product Catalog, About, and Product Description. Google Analytics can help you find out which are the most visited sections/pages on your site. Apart from Google Analytics, you can also use PercentMobile, as we saw earlier.
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Using an analytics service is one way to decide which devices you want to support. Another way is to use browser grade to know what category you should be targeting. jQuery Mobile has an awesome grid support chart at http://jquerymobile.com/gbs/. There is also a slide on jQuery Mobile that talks about the overall strategies regarding mobile browser web development at http://www.slideshare.net/jeresig/testing-mobile-javascript.
I have been collaborating with Jonathan Neal, and many others, on Smartphone frontend matrices. You can have a look at:
https://github.com/h5bp/mobile-boilerplate/wiki/Mobile-Matrices
It contains information about most Smartphones in the market, their screen dimensions, DPI, and operating systems.