Minimum SEO for a new site?

Question

Search Engine Optimization

What is the absolute and necessary minimum in terms of SEO for a new site, what should I pay attention to? The site is completely custom, that is, without any CMS. I don't need anything extraordinary, just the bare minimum.
That is, probably, to expose all html meta tags? What exactly? What else?
Where to add? Or will Google and Yandex index it themselves?
Site in English.

Answer 1

Make valid code, semantically correct.
other...

Answer 2

I will supplement the answer Vladimir Yurchenkov :
For starters, you will have more than enough. For each question there are a lot of detailed guides, since these are fundamental things.