Nephew hooked up the battery terminals wrong and now truck wont start. it'll crank but not fire. where do I start? we've replaced several fuses but still no luck.

 

What year, model, engine do you have? Which fuses specifically did you change?

 

See my signature.
Start with basics.

Got spark? Got fuel pressure? Hear, feel fuel pump run when you put IGN switch in "Start" position?
Did you verify that fuses were bad and that replacing them will fix the problem?
Got a multimeter?

 

Start by telling us what you are working on, like RAD4Runner said, then continue by getting a voltmeter and a test light. You probably have a blown fusible link, so check for power to your fuses with the key on and see what is getting power and what isn't.