The contact information for your utility provider should be located on the paper bills that your provider should have sent you. If you opted for paperless statements, or if you haven't yet made any payments, you can visit the website of your utility provider to view its contact information.
If you have moved into an apartment, your landlord should have provided you with contact information for the utility provider, but if you have moved into a new home, you may have to find the website. Not all utility providers operate in all areas, so you may have to change providers when moving.
Other methods of finding a utility provider's contact information include going to your city's municipal building and inquiring on the utility providers in the area or going to your state's public service commission website.