need to know

i have 1998 chevy z71, scan tool shows PO420.catalyst system effiency below threshold (BANK ONE).ANYBODY TELL ME WHAT I NEED TO DO TO FIX IT ,OR TELL ME WHAT SITE I CAN GO TO TO FIND OUT.THANKS

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.