Learn to box.
This is great advice for 2 reasons.
A. What most people are talking about when they say "fight", is some kind of unarmed exchange that has to do with your position on the social latter (breeding rights, tribal position
) . So learning to box is very relevant to this question. Learn to box, good answer.
B. If they do learn to box, they will learn that there is much more to fighting then boxing. And while boxing is a very useful skill, they would much rather use another skill set when facing real danger. This is the reason the military has stockpiles of weapons, but not boxing gloves.