The first rule of self-defense

Even if you have never taken a martial-arts class, you can certainly learn some basic self-defense techniques at any age.  In fact, you owe it to yourself to do so.  Many senior centers offer either martial-arts or self-defense sessions, as do many community education programs throughout the country.  If all you do is learn some basic strategies and techniques, great.  If you decide to join a school and do some serious martial-arts training, even better.
     Either way, the first rule of defending yourself from an attack is DO SOMETHING.  Now that sounds almost too obvious to mention, doesn't it?  But it's a fact that some people who are attacked do nothing.  Well, that's not quite true.  What they do is freeze.  They keep their hands down; they close their eyes; and they cringe, waiting for the blow to land.  And this, of course, can be the shortest path to the ER or worse.
     Okay, so what does DO SOMETHING mean?  First, it means being mentally prepared to defend yourself if a) an attack seems imminent and
b) it doesn't appear that you'll be able to talk your way out of the situation.  By being "mentally prepared" I mean accept the fact that you could be attacked -- even in a place as seemingly safe as a super market parking lot -- and therefore have a simple strategy in mind.  Among other things, being mentally prepared means staying aware of your surroundings, most especially any suspicious individuals who might catch your attention.  When in doubt, stay away.
      Second, DO SOMETHING means being capable of buying yourself some time while you yell for help.  If you're in a public place at a decent hour, help is usually nearby, and there's a good chance that someone will come to your aid if you yell for help.  But you certainly want to avoid being punched in the face or stabbed in the chest before help arrives.  Once it's clear that you're about to be assaulted, almost any defensive action -- something more than simply freezing and waiting for the blow to land -- may spare you serious injury. 
     Martial arts students learn a wide variety of blocks, but if you can learn only one technique, the high block is probably your best bet.  Although it's designed to protect against a blow to the face, the same motion can protect everything from your midsection to the top of your head.  Whether you execute the block flawlessly isn't terribly important.  If your life is on the line, fending off the first blow with a strong block may be enough to buy you the extra time you need.
     The best way to learn how to deliver a powerful high block is to attend a self-defense class or to sign up for a month or so of basic martial-arts training.  By practicing the basic blocks over and over again, you'll find that they become automatic.  And if you're ever actually attacked, you won't have to think about what to do.  You'll DO SOMETHING other than freeze.
     While you consider taking a formal class of some sort, you should look at this short video on how to execute a proper high block in taekwondo.  Different martial arts may teach slightly different variations on this block, but all of them will get the job done.  http://www.youtube.com/watch?v=FYADNvog3Dw