Basically, you'll be claiming a home office. Then based on the percentage of square footage you a renting, that percentage of the rent will be deductible as a home office expense, along with an equal percentage of the utilities (gas, electric, water) provided that specific utility is physically present and available in the space you designate. (So if there's no sink or toilet in the garage, don't claim the water bill percentage.)
It's important to note that you can *NOT* take depreciation on the home office, since you do not own the property. But the program can handle this just fine, provided you enter all the data correctly.