I bought a used 1998 Montero Sport it drove fine i added gas ans since then it smokes bad,, white smoke with water from tail pipe what have i done??? I was asked if i had put diesel instead of gas?? No i put gas in

Maybe a blown head gasket... Does the exhaust smell of coolant?

Overfilling oil may also create a major smoke cloud. May be difficult to tell if it's blue/black or white from inside but it would be blue/black if it were too much oil.

Maybe the gas that you put has some mix water... Or bad fuel system... Have some check up from the good mechanic