The width is measured in the middle of the door from one side or another. Measure on the inside of the garage so you can get the true width. To measure the height, put your tape measure on the floor and go to the top of the door. Find a level spot on the floor to measure from.
You will need to know how much room there is between the garage door tracks and the garage wall. A standard garage door needs 3.75 inches on both sides of the vertical tracks to operate.
The last measurement you need is to determine how much headroom you need. Measure from the top of the garage door to the first obstruction that the garage door must clear. A standard door needs 11-12 inches of headroom for clearance.