You need to know the following:

For a 27" or 700c wheel
Length of the steerer tube needed (measured from the top of the fork crown to the top of the steerer)
Length of the threaded portion of the steerer needed

That's mostly it. If you're trying to replace the fork on an old French bike, you have a few more things to worry about. If you want to fit a fender, it would be good to have eyelets on the dropouts.

Neal