You should start by thinking about your mobile experience first: a substantial amount of email opens now happen on mobile devices, and emails designed for mobile viewing have much higher click-through rates.
On to more general advice…
Emails should start with a single hero image that has a clear call-to-action.
Additional content should be organized in a single column below.
Use larger fonts than you think you need and make buttons touch-friendly.
Finally, take advantage of subject line personalization and preheader text to give yourself the best chance of earning that open.