Peon is right that both structure should be same to inorder to migratation to take place. In other words you should have same table and fields.
Your going to need a some kind of conversion script. This is going to be a script that converts the data in the joomla database to a wordpress format. Have you checked the wordpress support forums and asked there? With joomla and wordpress being so popular, I am going to guess that your not the first person to ask this question. Chances are that someone (maybe the wordpress developers?) has made a script that can do the migration for you.