I need help with creating redirects for 2500 '404' errors

I moved my website from Joomla to Wordpress in December and am now encountering high levels of '404' errors in Google webmaster tools.

I would like someone with experience of working wordpress as this the plugin to fix the errors uses Wordpress.

Speed of performing the task is not the priority here - it is making sure the redirect goes to relevant pages, ideally pages that exist under the main menus already on the site.