Truck will not start... Cranks over and over, keeps on cranking, back fires... Where do I begin to look for issue ???

had same isuue with a deawoo (i know try and stop laughin, whata pile of junk)anyhow. it started to bring me home from work, next mourning same thing. big back fire, muffler in pieces. turn out the timing belt had gone bad. my suggestion is to check the timing with a light, hope this helps.

There is a tsb for a no start condition or poor idle back fire etc. The cranshaft position sensor has been known to cause this

If timing turns out to be ok then check fuel pressure.