2.2L engine shaking and sputtering and spurting

not my car , the girlfriends sisters car , it doesn't see any maintenance, so here is the issue, the engine sputters and spurts at a idol and when you hit the gas it accelerates but you can almost hear a massive sequences of misfires , it sounds like the exhaust is plugged, i just had a new muffler put on so maybe it was put on backwards?? I'm going to change the plugs, and air filter , and fuel filter if it is easily accessible, in laymens terms "it runs like a bag of S#@T!!! "