The main tags
og:title, og:description, og:image, og:url, og:site_name and og:type describe the page. A separate twitter:card tag chooses between a small summary card and a large image card.
The image
Use a full web address and an image around 1200 × 630 pixels for the large card.
Try it
The Open Graph Generator builds the tags, and the guide explains where to put them.
Common misunderstanding
Open Graph tags change how shared links look, not how a page ranks. Networks cache previews, so updated tags may take a while to show.