problem

IAM USING A BMW 318I 1996MODEL,4 CYLINDER CAR. IAM NEW TO THIS CAR. ONLY 5 MONTHS SINCE A BOUGHT THIS. NO IDEA ABOUT THIS CAR. YESTERDAY I WENT TO THE LOCAL MECHANIC TO CHANGE THE HEAD LIGHT BULB. HE CHANGED IT AND AIR CLEANED THE ENGINE AND BATTERY WATER ALSO ADDED. HE CLEANED THE AIR FILTER ALSO. LATER I STARTED THE CAR AND DRIVE FEW FEET THE PROBLEM STARTED. ITS RPM WAS GOING DOWN AND THE ENGINE VIBRATING AND BLACK SMOKE COMING OUT OF EXHAUST. STILL THE PROBLEM EXIST. PLEASE HELP, IAM TERRIFIED.