SEO 101 Suggestions - where to start with DIY SEO?

I am starting an ERP/Accounting Software selection and implementation business. Inbound leads through online searches are important to my year 1 success - I have limited startup capital and while I might choose to spend it on the website + SEO, I would rather DIY it at the start. Having spent my career in accounting and software though SEO is a foreign language to me. I know there are tools to learn what ranks and how to optimize content but I don't know which tools are the best. I know keywords and tags are important but I don't know how to make sure the content I write is "optimized." I know I need backlinks, I don't know how to get them. Is there an online course? A holy grail of SEO 101? A popular youtube channel I could/should start with to learn this wizardry and lay some foundations and get the boat moving in the right direction before spending what limited startup cash I have?