When it comes to search engine optimization, there really is no most important step to achieve results. But however, there is a very first important step and that is figuring out what keywords you are going to use for your website. There are many other factors regarding importance as in quality content, quality backlinks, social media, etc. But all that won't matter much down the line if you pick the wrong keywords to begin with.
Keyword research is the first process you must go through when setting up a website. Your url must have the right keywords. For example, if you have a website that sells stickers, the word "stickers" must be in the url somewhere. There are many online tools available that will help you with this. I usually prefer going on Google Adwords and look up keywords there. The trick is to find related keywords to your niche that has the least competition with the highest search rate. Once you have this handled, everything else that comes to SEO is secondary.