Hopefully i can help..
You say you replaced a sparkplug..so have you narrowed the missfire down to certain cylinder,what fault code did you aquire? or have you replaced "all" 4 plugs?
What make of ignition coil does it have?
What engine size?
Any warning lights on the dash? any tell tail smoke from the tail pipe?
Has this been a sudden issue or one thats got progressively worse?