Almost all digital cameras have the ability to turn on the flash or turn it off. Most people leave it set on automatic all the time. If you are in the above situation, put the sun at your subjects back and turn the flash to on. You will usually see a button with a lightning bolt. Just press that until you see the flash turned on. If you see a line through the bolt or the word "auto" keep going. Now, the flash will fire when you take the picture. It will "fill" in the subjects face and reduce the harsh shadow. Hence the name "fill flash." Simple huh? Give it a try next time you are out and leave a comment to let me know the results.
If you want even more tips on shooting in the sun, there is a great article here.