Farther means "more distant" and is usually measurable, as in these examples:
Is El Paso farther from Dallas or Los Angeles?
John decided to climb farther into the cave.
Sometimes farther can be used more figuratively, such as:
My best friend and I drifted farther apart.
Further is a more abstract term meaning time or quantity ("more" or "additional"), as in these examples:
Katie developed further complications.
Jim needs to research the subject further.
In writing, you'd be okay to use further any time you don't apply it to a distance.