As above, you will report rental income and expenses on Schedule E. The usual way to determine expenses when you rent a part of your house is to divide the area of the part in which the renter has exclusive use by the total square footage of the house, and use that to prorate expenses for things like utilities, mortgage interest, insurance and taxes. For more information see this IRS publication: http://www.irs.gov/publications/p527/ch04.html

In particular see the section on Renting Part of Property which says, in part:

How to divide expenses.   If an expense is for both rental use and personal use, such as mortgage interest or heat for the entire house, you must divide the expense between rental use and personal use. You can use any reasonable method for dividing the expense. It may be reasonable to divide the cost of some items (for example, water) based on the number of people using them. The two most common methods for dividing an expense are (1) the number of rooms in your home, and (2) the square footage of your home.