Although some people think that Mercola is a quack (and I admit that my opinion of him has declined), I think this article has good info: High-Intensity Interval Training 101 | HIIT Benefits
I mean, in general, what you want to do is run really fast for either a set distance (like 200 meters or 400 meters) or a set time (like 30 seconds), and then rest (walk/jog) for some amount of time. Usually, if you're doing time intervals where the work interval is less than a minute, a good place to start is to use a 1:3 ration for work:rest. So if you sprint for 30 seconds, you would rest for 90 seconds, then sprint another 30 seconds, rest 90 seconds, etc. You should do between 4 to 8 intervals.
You can also do these on a stationary bike or rowing machine if you don't feel like running.
ETA: Mark has said somewhere that you should eat more carbs the day before doing sprints in order to fill up your glycogen stores. He said something along the lines of: there's no point to doing a glycogen-depleting workout if you don't have any glycogen to deplete or something like that. I'll try to find it.