There are a lot of sensors, so I assume you mean the fingerprint sensor.
Wash your hands, dry them and let them warm up. Then stand with your hands at your sides for a minute or two, to let some blood get down to your fingers to plump them up. Then try. (That's usually the cause of the "clean the sensor" message - not a dirty sensor, a defective fingerprint.)
If it's really a dirty sensor, a Q-tip dipped into rubbing alcohol (7-% is fine, but 90% is also okay), rolled around the inside of the neck of the bottle to make it damp, not wet. A few swipes of the sensor will clean it. (About the only cause of "dirt" that causes the sensor to fail is skin oils, and alcohol cleans them.)