It backfires and will not start but it used to run?

How does the engine sound when you crank it over normal? If so, it sounds like you have a fuel pump that is out. To be sure this guide can help you check the fuel pressure and I have instructions in the diagrams below to help change the pump out. If you are unsure of the engine condition and can shoot a quick video with your phone so I can hear the engine, that way I can tell for sure what's going on. You can upload it here with your response.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

It should be:
FUEL PRESSURE
Ignition ON Engine OFF .................... 62-90 kPa (9-13 psi)

Check out the diagrams (below). Please let us know what happens.

No, it's getting gas because it catches fire right where the throttle opens when you hit the gas pedal. I think it's called the fuel injectors and when it hits it'll catch fire it's my dad's old truck, he gave to me, and he said that it needed to be adjusted to the first spark plug the distributor anyway, but the battery I have is no good and don't have a battery charger, so I'm waiting until payday to buy a new battery.

Oh, and it's a 2500 series truck with a 350 and 6 quart oil pan and it's a 1991 2wheel drive.

Gotcha, I would clean the throttle body including the IAC passages, then I would install a new IAC (idle air control) motor. here is how in the diagrams below.