my 2002 dodge ram has a missed fire cylinder?

[AlejandroZ]

my truck started with the check engine light on and a noice inside the engine so i took the truck to auto zone they told me it had a missed fire cylinder #5 so we called a mechanic he changed the spark plugs and changed the oil i had never change the spark plugs before and almost never changed the oil, so after he left the about 2 days later the truck started throwing grey smoke from the exhaust pipe and the truck shakes when ever the truck is in park or in a red light what could it be what can i do