toilet running intermittently
The flap may not be seated well at the bottom of the tank.  So the first, and cheapest step is to replace the flap.  However, if you've been adding tablets or chemicals to the tank, those can corrode the PVC fitting (flange) where the flap seals.  In that case you need to replace the PVC flange. 

The smell is a second issue.  That has to do with sewer gas getting into the room.  The other answers suggest the wax ring.  That might be right.  Does the commode wiggle when you sit on it?  If it seems steady, it might come from a dry trap elsewhere in the room.  If you have a sink or tub that isn't used much, make sure you periodically pour water down them.  This puts a gas barrier in the trap, without which, gas comes into the room.