All rules are made to be broken in art. Don't believe me? Pick a rule and go to Borders and look at big glossy art books or magazines. See how long it takes you to find great photographers breaking someone's cherished rule. Rules are a great way to get started when you have no idea what to do.
Here's a few starter ideas for composing:
Don't put your subject dead center.
Fill the frame so it looks / feels right.
Be aware of the background.
Are weird things coming out of the people's heads?
Is the background being used to enhance the subject of the picture, or at least not detract?
Do things feel crowded, or is the subject lost in too much space?
Are distracting things happening around your subject that draws the viewer's eye away from your intended focus?
Have you considered what would happen to your composition if you moved a little to your left or right, or gotten down lower or up higher? Sometimes the slightest change can make all the difference - explore your options!
What about shooting it wide angle verses telephoto or vise versa. A wide angle will exaggerate the foreground and throw the background farther back. A telephoto will give narrower angle of view, compressing the foreground and background closer together. Often going to a telephoto instead of a wide angle will save you from including things on the edges of the background you didn't want.
A great way to learn is to try shooting the same subject using wide angle and telephoto, while trying as many different angles as you can to see what works and what doesn't.
Good composition is largely subjective. Very experienced visual artists can have radically different ideas about what makes for good composition. Most subjects can be successfully composed in an almost infinite number of ways. Most photographers with time and experience will naturally fall into their own way of composing. Study the work of great photographers.