Measure the door from side to side, and top to bottom.

If you buy a prehung unit, it will be listed as a 36" by 80" door. The rough opening---the hole where the door is installed will be larger to allow room for the door frame. Something like 38" by 82".

You will need:

A book on replacing an exterior door.

Long level---minimum 4' long---6' is better.

Shims---I prefer the cedar ones(traditional) but composite shims are available.

3" long screws---to replace at least one screw in each hinge on the frame side. That will lock the door frame into the house frame.

Tube of thresh hold sealer---for sealing under the thresh hold. The specific sealer never hardens, which keeps the space unter the plate sealed.

I use finish head screws to install doors. You push back the weather stripping and drive the screws through the door frame into the house framing, using the shims to keep the frame straight and plumb.

There is much more to the job, so read the book carefully. You can get one at a home improvement store.

Awesome...Thanks for the info!

I will definitely be going the prehung route!

What Mac said! When you measure the existing door it will be a little smaller, maybe 35 3/4" that's a 36" door.

You'll also need to know what "hand." An easy way to tell is stand in the door opening with your back against the hinges. Your hand that moves in the direction of the door swing is the "hand". So if you left hand moves, it's a left hand door. You'll also need to know if the door opens in or out. So you'll be buying something like a left hand, in-swing door.

Don't buy the little pine shims they sell in every home center right next to the doors. Wander over to the building materials department and buy a pack of cedar shims. They come in a bundle much bigger than you will need but they make great kindling if you have no other use for them. After you've installed the door, you take a utility knife and score the shim. Whack off the remainder with your hand. Pine doesn't whack very well. :-)