Advice on Developing First WP Theme [noob alert]

Hullo r/Wordpress!

Like many before me, I have a good knowledge of HTML/CSS/PHP and I've just started developing my first theme; I'm fine with that side of it but I don't know how to implement a certain feature that my client wants.

Here's the mockup that will hopefully help explain what I'm trying to achieve.

Under the 'Featured' heading my client would like to be able to add their own post with the image and a summary. As a complete noob, where do I start with this? Is there a way of telling a post (through metadata perhaps?) that it's 'featured'?

Thanks for any advice or pointers you give :)