Do I have to wait until I have great content to start doing SEO for my SaaS startup?

I just started the website for my SaaS startup, and I was wondering when should I start doing SEO? And also, do you think it is relevant to do it without having a high-quality website with great content? If yes, how can I do that?

Some friends told me to start submitting my business to online directories while others told me to focus on having high-quality content before anything in order to start doing SEO, so I don’t really know which advice to follow.