Don't dive straight into adding ARIA roles. You're likely to mess everything up.
If you start fixing an inaccessible product without knowing what you need to know, the only thing you'll accomplish is creating more work for you. The result is likely to be months fixing problems you could have avoided in the first place.
Mind you though, that doesn't mean you need everyone to become an accessibility expert. You just need everyone to know their part.
The simplest way is to break required knowledge down by role. And to make it specific.
Designers need to know about colour contrast, focus states and touch target sizes. That's where most visual accessibility issues start.
Developers need to write semantic HTML, manage keyboard navigation and use ARIA properly (or better yet, not at all unless they have to).
Content writers need to write clear headings, useful link text and alt text that actually describes what's in the image.
Testers need to test with a keyboard, check with a screen reader and verify forms make sense without seeing them.
Product owners need to include accessibility in acceptance criteria and make sure it's in the definition of done.
And please, keep all this simple enough so that people actually remember it. Putting it somewhere visible can't hurt either.
The foundations aren't everything, sure. But without them, everything else falls apart. And you'll spend your time fixing shit that never should have been broken.
Foundations are the third thing on my list of 10 things you should focus on.