How to evaluate seo of a website?

Hello everyone.

I have recently noticed that my site has a low ranking in search engines. I want to understand how my company professionally and conscientiously optimizes the site. I want to identify shortcomings on the site and take timely measures to eliminate them. I do not understand the problem of my site. I understand that I need to deal with SEO. However, could you explain it to me, how I can evaluate seo of a website? Please write step-by-step instructions.

I hope you help. Thanks in advance.