If i rent out a house but keep the garage as personal use would that be considered as renting only a portion of my rental house?

Yes, that's correct. The rules define that any day of personal usage should be accounted; see "What is a day of personal use?" at the IRS rental publication  HERE.  

You can choose to let the software calculate your expenses, so that you only need to enter your expenses for the entire home.  The software can then calculate your rental expenses based on the square footage percentage of the home to the entire property (home + garage). Also, if you make these selections, the software will allocate the personal portion of property taxes to your itemized deductions and same with the mortgage interest (if it doesn't exceed the second home/1.1 million limitations). 

To enter this:

1. Click Take me to my return
2. Select the Federal Taxes tab and then click the Wages & Income tab below it 
3. Scroll down to the Rental Properties and Royalties section and click the blue hyperlink Show more
4. Click Start/Revisit next to Rental Properties and Royalties (Sch E)
5. Next you'll be asked about your real estate involvement and then you'll be at a summary screen of your rental property(ies) (or you can start setting up your rental). If already started setting it up, click Edit next to the property.
6. If you already setup your property, you'll be at Review your XYZ Rental Summary. From this screen, click Edit next to the Property Profile. On the screen titled, Do Any of These Situations Apply to This Property? be sure to select I rent out part of my home.  The next screen will recommend letting the software allocate your expenses and you'll need to select Yes to let the software do so. Continue answer the questions.
7. Back at the Review your XYZ Rental Summary you'll be able to enter all rental income and expenses, making the selections from this screen.