What's up man, I'm in NJ too. Where are you servicing? And as for your questions
1. What exactly do you want a DEP number for? Hauling Garbage?
2. You will need General Liability Insurance & Commercial Auto
3. You will definitely need commercial plates on your trucks, as well as some sort of lettering. Just go to your local MVC and tell them you need commercial plates once you have your commercial auto policy.
4. You must register with the state to do business at: http://www.nj.gov/njbusiness/registration/