Need help with What to know about Seo before paying someone to do it for you

I'm Workin on a online jewelry store and I need help with Seo. Should I learn it myself or would it be better to let someone who knows do it. If I commission someone to do it for me, what should I know in order to know that I'm getting what I'm paying for?. Is there anything in particular that I need to worry about? Thanks for helping me out.