99 firebird obdII error help

i did and obdII diagnostic test and had 2 errors: P0300 Random/Multiple Cylinder Misfire Detected and P0420 Catalyst Ststem Efficiency Below Threshold(Bank 1). Any info on the problems or how to fix them or what to look for? Any help is appreciated. Here's a pic of the car... www.geocities.com/theonetekkenmaster/2.jpg