Display recent posts horizontally on web but vertically on mobile

Hi
Could you please tell me how to display extracts and an image from my recent posts (preferably from a specifically selected category) horizontally on my homepage and vertically on a mobile. I am using TwentyTwelve. I know how to display them horizontally with CSS using float: left – please correct me if I’m wrong! But I don’t know how to change this to be responsive.
Any help would be much apprecaited!
Thanks a lot.
Tash