There are two basic ways to do interval training:
Measured intervals. With this type of training, you work hard for a measured period of time or distance and then recover for a measured period of time. This Beginner Interval Workout offers an example of measured intervals.
Varied Intervals. In this type of training, you simply work harder for as long as you can and then recover for as long as you need to get ready for the next hard interval. For example, if you're walking or running outside, you could choose something in the distance and sprint to it or walk/run up a hill as fast as you can and walk down to recover.
There are other great resources out there for guided interval workouts to really challenge you. My favorites are Cathe Friedrich's Imax 2 step intervals and Cardio Coach's Guided Interval Workouts.

