Metadata is an important part of a website. If you are getting poor scores from website graders and don't see any place on your site to add that information it likely wasn't implemented by the team that built your site.
It's good you've noticed this and it's worth taking the time to set up a way to manage the metadata on your site. The best way to do this in a comprehensive and maintainable way is with an SEO plugin. Once you have things setup, you should no longer need the assistance of a programmer to manage the majority of your SEO needs.
For Craft, at this point in time, there are two good SEO options that are very comprehensive in the types of metadata they manage:
My firm is behind Sprout SEO, so my preference is with it, but SEOmatic is made by a respectable developer who I would recommend as well so it's really a matter of your preference on which one works best for you.
With either plugin, once you have it setup, you can focus on your content and the SEO plugins will worry about generating and outputting the metadata you need on your pages. If you decide to refine or grow your SEO strategy, both plugins also have ways to give you additional control over customizing the metadata further, but that's probably something that can be left for another time in your case.
Leveraging a plugin, it will likely take a developer 2-4 hours to implement a good SEO baseline on your site. For a custom or more advanced implementation, additional planning and implementation will be required.
Please take this estimate with a grain of salt as every custom website is unique and if your original developer didn't set up any metadata that could be a signal that the rest of the project also has some non-standard things in the setup that would take the next developer more time to figure out. If you have a large site or advanced content, it can also take longer to customize metadata for special considerations, specialized content types, or your particular goals.