Misfires

My 2002 Bonneville was running fine when I noticed it was not running smoothe. I could feel it misfiring and has little power. This came on all of a sudden and very noticeible. When idling if you put it in drive with the brake on, the engine shakes. O'Riley's Auto pulled a few codes out but the one that I suspect and need more info on was the message "multiple misfires". Where do I start? What do I look for? In what order? Plugs, wires, sensors, etc? There was also a code for the powertrain Control Module which I know nothing about or it's location. Thanks for any information to all who help. FYI my last problem was fixed with a new crankshaft position sensor. I replaced that after reprograming the keys with the computer failed to fix the problem. The car (04 Bonneville) has not failed to start since then. Mike