Are you thinking of replacing an exterior door?  There are so many designs and styles to choose from these days, but the first deciding factor must start with the material!  Based on what part of the country you live in, will be a huge impact on what materials would be good to choose from, such as steel, fiberglass or wood.  For example, I live in MInnesota.  Here we use all three types.  If you choose wood there are major prefinishing instructions.  If you are going to paint you should use several coats of oil base primer due to the fact of the concept, "water and oil do not mix.