well, looks like my dad was right about THAT....

When I was small(er), my dad set about trying to teach me with the basics of fighting. As I was a year younger than everyone else in my class, and small for my age, I was pretty consistently getting picked on.
His advice was short, sweet, and to the point (I have only two of those three qualities, myself):

1. if you're going to punch, go for the face: eyes, nose or throat.
2. if they're too tall for that, punch or kick in the crotch (boy or girl, it will hurt)
3. follow up by knocking them down before you:
4. run away.

This is actually VERY good advice. There are lots of strike points on the human body--and some of them are especially good. Just look at this:

I mean, there are some fun spots you can use to mess someone up, right? And that's not just "vital points," in that image, either, you've got some nice "nerve points," too. That seems like a LOT to choose from... almost too many, really.

But here's the thing, a good many of those points don't work all the time.
I, myself, don't respond to pressure on my Median Nerve. If someone has good muscle control, or a nice layer of fat, or maybe even a fairly thick sweater or coat, you won't get the same effect on body strikes to the ribs, diaphragm or kidneys.  Boots or shoes can prevent damage to the instep or Achilles tendon.

So what does that leave? Well, there's always the face. And here's the thing about hitting someone in the cranial area: your brain does NOT like that. A hit to the eyes will make them water, so will a good hit to the nose. Strikes to the temple or throat can possibly kill someone. They are vunerable areas, and can (and should) be exploited if your life is on the line.

But what if they're too tall? Yes, this can happen. I've sparred with people that I would literally have to stand on a stool in order to hit in the nose, and trying to hit them in the nose just looks stupid and wastes a lot of my energy. So I don't try to hit them in the nose... at first. First I find a way to bring them to me... say... a kick to the groin (or knee... or even a good roundhouse to the abdomen). Then I go for the face.

And usually that accomplishes step 3 -- knocking them down. The goal here is to get to step 4. If you can knock someone down, you can make it harder for them to chase after you, because you do NOT want to have a sustained fight with someone outside of the sparring ring. A well placed kick to the groin or knee will certainly help here. But often so will a good elbow strike to the jaw, the throat or the temple. 

The point is, don't make this too complicated. It's not about mastering the Five Point Palm Exploding Heart Technique... it's about doing something simple and effective that can get you out of immediate danger. And in that respect--dad had some pretty good advice.