Is cable/internet considered a utility when figuring out utility expenses for renting out part of our home?

Yes. But only if it's shared with the paying tenant.

It is shared with the tenants. 🙂

How can u add these expenses if u have no roommate. I pay rent., late fee, electric n internet bill

If you had no roommate in 2015 that paid you rent, then you're not a landlord at all. So you would have no rental income to claim, thus no rental expenses to claim either. If the SCH E is a carry over from 2014, then you can either report zero income and zero expenses, or you can report that the rental was "removed from service for personal use" if you stopped renting the space in 2015.

I have a similar situation, but I rent out a garage apartment which is next to my house.  The internet service and TV streaming service is shared between both the house and the garage apartment.  I use both services every day.  Renters use both services when the garage apartment is occupied.  My question is how do I calculate how much cost for each service to allocate to the rental (i.e., deduct from my taxes).  Certainly both services are not fully deductible since I use them for personal use too.

use any reasonable method you can come up with.  there is no correct answer in your situation.  in theory it could be the number of hours the tenant uses the service as to the total hours of usage but you probably don't monitor the usage.