Hi
There are three key parts to it generally. This is very general so ask for detail on any part. The first is technical seo, making sure your site can be found by search engines like google. Ria has mentioned that. To that end the below technical site audit guide from moz is a gem. Lets you tick off where you are at. A website checklist! Two items I would immediately look at are "mobile optimization" & "home page loading speed".
https://moz.com/blog/technical-site-audit-for-2015
The second and third parts flow together in some ways - backlinks and content creation. There is a lot of information out there about content so you will find plenty of info. On backlinks - another great link. https://moz.com/beginners-guide-to-seo/growing-popularity-and-links
If you feel comfortable with your site after reading both links, you have all covered - you are well on your way to optimizing your ranking.
Hope that assists. Ask away for any clarification.