Why is smoke entering my home through the fireplace?
Damper closed - most likely.

Chimney blocked - while "damper closed" is a self-correctable version of this, if the fireplace has not been inspected there may be anything from bird nests to parts of a chimney in serious disrepair blocking the flue.

SO - before you become a statistic (of the chimney fire sort) call a chimney sweep and have the flue inspected to be sure it is in safe condition to use, and find out where the damper control is (often a lever or chain hanging down in the top of the fireplace.)

Another possible problem, particularly common if the fireplace &amp; chimney is on the outside wall of the house, would be an excessively cold flue - sometimes curable by putting newspaper up into the throat of the fireplace and lighting it (before lighting the fire), to get air moving the right direction (up and out.)