How can I reset the ecu on my car so that the light on dash is gone

If you are referring to the car's computer and the light coming on is the 'check engine' light, then you need to find out WHY the light is coming on. If you don't already have one, buy a basic OBD reader (+/- $20 on Amazon), plug it into the OBD port (usually under the dash near the hood release lever) and see what the OBD error code registers. You can then decide what to do. My unit also has the facility to delete the error codes, but when the computer goes through another check cycle, the light will come back on.