You can figure it two different ways -
1. Percent of square footage used for business purposes. Example: You have 1000 square foot house, and you use 100 sqare feet of it for business purposes. You effectively use 10% of your home for business purposes. If your rent/mortgage is $1000 a month, you can deduct $100 (10%) of your rent, as well as 10% of your utilities with the exception of the first phone line in the house.
2. Percent of room usage. Say you have 10 rooms in your house, and you are using one of them for your home office. You are once again using 10% of your home for business so you can deduct 10% of rent/mortgage as well as utilities(minus first phone line).
I'm not sure how a garage/parking works out. If it's significant, then you'll want to consult a tax account on it.
Any more questions, I'll try my best to help out.