So that you're not relying on your memory when you go to the hardware store, make a rough drawing of it, from the outside or exterior view. Mark on the sketch where the hinges and door knob are this will tell you which way it hangs. Label whether it's a left- or right-handed door; if the hinges are on the right, it's a right-handed door. Also write down whether it's an in-swing or out-swing door. Most exterior doors are in-swing doors, meaning they open to the interior of the home.
Now it's time to measure the width, height and depth of the door jamb which is the wooden frame that surrounds the door. Don't measure the door because it may have been fitted poorly in the first place and may not be a true reflection of the size of the jamb. Once you've got your drawing and measurements, you're ready to go and choose your new front door.