Without a model number for your mower, we can only offer some general tips. Here are some suggestions that may help:
Bad fuel (or water in the fuel) can cause the engine to smoke. Try emptying the gas tank and filling it with fresh gas. This may eliminate the smoke (often a white color of smoke).
If the oil has not been changed in a while, try changing the oil in the engine. Make sure that the oil is not overfilled. Overfilling the engine with oil can cause it to smoke (often will be blue smoke).
Check the air filter on your engine. A dirty air filter can cause the engine to smoke in some situations.
The engine could have worn rings or other serious problems that would cause it to smoke. These types of problems would normally require a service technician for repair. Here is a link for the Sears Service website in case you need it:
Sears Home Services
If you need more help, resubmit your question with additional details and the full model number of your lawnmower.