Have a look at the warning lights panel. Any new (unacknowledged) warnings will be blinking. You can silence them with the master caution button on the UFC.
If by 'APU switch' you mean the APU generator switch on the same panel as the battery switch, then the first warning you're hearing is that the APU generator is enabled but not running/active (because the APU hasn't been started).
Also don't forget to enable the inverter when you're powering the jet up. This is required for power to some of the engage gauges.