Lots of things to do For SEO.
Firstly you need to check your site structure whether it is SEO friendly or not. Check URL structure.
Then after create a Meta information for all the pages and also create a good quality content for all the pages. Also create Robot.txt fie, XML Sitemap, W3C validation, etc.
Add your site in Google Analytic and webmaster tool and put code of analytic and webmaster tool in your site.
Then after you can start SEO OffPage activities and another SEO Work.