What to do if lawnmower smokes when turns on?

As long as you keep the oil level full you can run it until it gets so bad you get ashamed of the smoke and buy a new one. That make take years. No risk of explosion. 

Most common reason on a four cycle is the cylinder and rings have gotten worn or scored. Damaged air filter can let dirt get in and accelerate wear. On a two cycle incorrect oil ratio would be the first guess.

Wrong forum. go to the maintenance forum in the Lawn & Garden category. This is Cars & Transportation. A mower doesn't qualify as either. 

It all depends.  With two cycle engines there will be smoke if the fuel/oil mixture is cold or the ration is wrong.  with a four cycle engine, oil can get into the combustion chamber if the mower is titled the wrong way.  be sure to use a fresh spark plug and clean filter.  it will note explodel.

Turn it off and get a new one. If you keep using it, it will blow up in your face Use logic.

Check your air filter, other than that check the oil regularly.