Well, the quickest (in terms of initial dev time), would be to just use media queries or any of the tons of grid layout libs like (http://foundation.zurb.com/) or bootstrap. This also works great for small sites, or sites where you don't really care too much about optimizing for mobile - you just want something that looks decent and works with minimal extra effort. But in the long term if the site is rather large, (in my experience) it'd be better to consider building a different app for your mobile version. This lets you filter and modify what you send over http responses for mobile (so the images are smaller, certain sql queries don't get run, etc).