I have already covered installing shingles in this shed building post.
The only difference is roof size.
Rather than three tab shingles I strongly recommend using some 30+ year architectural shingles because they are more than worth the difference in price.
Since I started using the more expensive shingles as standard and not an option on my sheds my roof related service calls have dropped by 90%. A few extra dollars now will be money well spent.
Here is a link to a post that explores installing metal instead of shingles. Please take a look to see if metal might be an option on your new garage.