As Saecaost said... You will need to study the DB design that Wordpres suses to determine which tables and fields you will need to fill in. As we don't know your existing DB structure it's impossible for us to say what data from your old system should go where in the WP system.
But, as a hint, wp_posts is the main content table. From there wp_postmeta holds extra information as well. Good luck.