Truck will crank, but not fire

Truck sat for over a year (travesty). It now has fresh gas and battery. I tried cranking it, and the first time, it tried to fire, then nothing. All subsequent cranks are fruitless. I can hear the fuel pump and it' s relay click, though the pump sounds a little labored. Truck was running good when it was parked. Nothing squirts out of the valve on the fuel injection rail. Obviously it seems to be fuel delivery, but where is a good place to start, and where are things like pumps and filters located? Also open to any other suggestions! Also, what is that switch (disk that pops up) under the battery?

Thank you very much.