First step get a code reader on it and see if there are any codes set, if there are that may narrow down where to look for the fault. Usual cause is loose or dirty airbag connections under the seats, with ignition off disconnect and clean the connectors then plug them back in ensuring they are a good tight fit and clipped to the seat bottom. If you turn ignition on when the connectors are disconnected it will set an airbag light that will reqire clearing with a code reader.