Custom Post Layout

Hello Everyone!

I’m wondering what sort of html/css I need when doing a custom post layout.

I want to have the module show just one post, fixed width, width with 3 columns. The right column will be the featured image, the middle for the excerpt and then the right column for some other (possibly custom field) content.

I want it to be responsive of course but don’t know where to start with that.

Any help will be greatly appreciated!