How do I know if a door is a right hand or left hand door?

Open the door and stand with your back to the hinges. Is the door on your right side or left side? If it's on your left side, it's a left hand and if its on the right, you have a right side.

Here's some info for you so you can measure your exterior door. You can also take a picture with your phone once you have measurements and bring it with you to the store where you plan on buying your new door.

http://videos.homedepot.com/detail/videos/appliances/video/3711799232001/how-to-measure-for-a-new-prehung-front-door

Here's how to measure properly! https://www.wikihow.com/Measure-the-Size-of-a-Door

Michelle,

If you are just buying the door (a front door slab) as opposed to a pre-hung door, which has the frame as well, you will have to most likely reset your hinges. Check to see how the hinges will be installed on the new door. A door slab does not come with a knob and lockset either and your old one may not work in the new door. Along with moving the hinges, you may also need to move the strike plate. The new door may or may not be weatherstripped. Make sure you know what you are up against and if you have the tools you need.