I have a 98 Ford Tarus LX, I change the spark plug on the car and now it is blowing smoke out the tail pipe, what could cause this? I thought a vacuum line might be the problem, could you assist me with advise.

The plug was fouling, when you changed it, it started working again. You have either bad rings or valve seals on that piston. You can buy an anit-fouler from most parts stores which will keep the plug from fouling, but it will not cure the underlying problem. In order to find your underlying problem, You will have to use a compression tester. If the cylinder has less than 80lbs. of compression, the rings are probably bad, if it has good compression then it is possible the valve seal.

The problem is more than likely going to be rings if it smokes all the time.
Bad valve seals typically smoke when you start the engine.
Either way, you will have to have a mechanic fix it, knowing is only half the battle.