If the smoke is bluish-grey in color, oil is getting into the combustion chamber. The oil sump is probably overfull - drain some oil out to where the level is within the marks on the dipstick. Clean the spark plug.
If the smoke is black, the spark is not igniting all the fuel in the combustion chamber. Three things to look for (1) dirty/clogged air filter; (2) fouled or not properly gapped spark plug; (3) carburetor adjustment.
Remove the spark plug and look for DRY carbon build-up (fuel problem) or a WET carbon build-up (oil problem) and go from there