Do a visual inspection of the brakes. If they are good, it sounds like the previous owner changed them AFTER the sensor told him to. You will need to buy new brake wear sensors and replace. There are only two of them, one on the front and one on the rear, mine were on the driver's side brakes.
To install, take the caliper off and spread it open, clip the sensor to its proper spot on the back pad, tighten everything back down. I suggest if you're doing this to go ahead and put some antisqueal sealant on them. In the DIY section youll find steps on how to do the things I said.