What to look for when trying to purchase SEO

I am trying to have my online cannabis store in canada grow. I figured SEO might be a next good step. Before purchasing, what should I be looking for and how do I see results? I have access to google analytics and google webmaster.

I was thinking of signing up for trial here https://www.blackhatworld.com/seo/d...links-from-7-link-free-trial.1062927/page-113 but what do I pay attention to, how do I know if it's working?

You post to the appropriate sub-forum.
But first you should search the forum as this has been discussed and some even posted some guides about it.