I assume that it is black smoke coming from the exhaust, which is indicating that the engine is burning excessive oil. The reasons for this would be:

1. That oil is getting past the piston rings, indicating that you have excessive wear in the engine cylinders.

2. Oil is getting past the valves (there are sealing rings to prevent this) and if these seals are unserviceable then again, oil is getting into the combustion chamber.

Of the 2, the valve seals would be the cheapest to repair, as if your cylinders are worn then you may be looking at a replacement engine.