2002 dodge ram has a missed fire cylinder?

[Lorenzo B]

i have a 2002 dodge ram that started with the check engine light on, after a while the truck started making a weird sound on the engine so i took it to auto zone they told me it had a missed fire cylinder so i called my mechanic and he changed the oil and the spark plugs after 2 days the truck started shaking and blowing alot of white smoke out of the exhaust and it has a weird smell could that be of the missed fire cylinder or what and how much would it cost