I Have a 2000 Express 3500 Bus that wont start.

Check for voltage drop on the main cables. Usually a bad starter will give you one loud click, but nothing sounds like a voltage problem.

You're not hearing any clicks, so I wouldn't immediately suspect the starter/solenoid.. If everything is normal when you put the key in the run position, and in the start position the lights don't dim and you hear nothing, the battery might be very low; like 3 volts. But I'd be more suspicious of the ignition switch.

In a 22 year old truck it's probably the ignition switch