Need help sorting out utility bills!?

I'm a second year student, and I've just moved into some student housing. I want to organise the utility bills, but don't know where to to start! The previous tenants were with Southern Electric for both Gas and Electricity. I want to change the company as I have found cheaper deals. How do I go about doing this? I've got electricity and gas at the moment, but I don't know who I'm paying it to. Please Help!