All the answers above is correctly, you can read it will be definitely helpful for you and my answer is also will be same.

Before you start just check the status of web page. What is its place and what type of work is done for this web page so after all verifying then plan an approach for any web site. Set your objective and period of time for this web page and then make a health seo strategy for site.

Starts with social media.
2.Article submission.
3.Guest posting etc

Do the following,

On page optimization
Off page optimization.

If you have more clarification on this please contact nearest seo training institute