The first step to measuring a garage door for your home is to measure the width and height, which is the measurements you'll need to report to the garage door installation person. But next you need to measure the 'side room', or the space in the room along the front wall that is not taken up by the door. Some garages don't have much, but you'll need at least three to four inches along the side of the opening in order to have space for the vertical metal track. (If you have an old extension spring or torsion spring. You'll need closer to five or six for the newer extension spring systems, though.)
Next you'll need to measure the headroom which is the space between the opening of the door and the ceiling of the garage. The top of the door opening is often called the 'jamb header', and you'll need at least ten inches to a foot of headspace to fit in the opened door. Even more if you plan on installing an electric garage door opener. However, if your garage doesn't have sufficient headspace, you can always invest in special equipment from the garage door service people.
Lastly you'll need to measure the 'backroom'- the distance between the opening and the back wall. It need to be at least 18 inches longer than the door is high, even more for an electric opener.