The biggest rookie mistake is thinking SEO is only a technical game, technical SEO is important but understanding search intent, content quality, competition, backlinks and user behavior is equally important. I would start with one small website, learn how to perform proper keyword research, optimize content, build a clean internal linking structure and track everything through Search Console and analytics. Also dont expect instant results or make too many changes at once because SEO takes time and you need to understand what actually caused the improvement or decline.