any trouble light codes? Likely dirty flame sensor or clogged air filter (when did you change them last?). Could be other things however. Google your furnace brand/model and flame sensor cleaning; you'll find hits for youtube likely that will show you how to do it.
how long does the flame burn for ? if it runs for 3 to 5 seconds i would agree with the flame sensor being dirty but if it runs for more then 45 seconds then the furnace could be kicking off on high limit due to a shot fan motor.