If your subject is outdoors, and there are shadows falling across his/her face, then use the fill in flash. The technique is the simplest way to shoot great images and is basically self explanatory. Before focusing for the picture, turn on your flash and let your camera do the rest. Most digital cameras are already set for this type of outdoor photography as it is just a matter of getting to know your camera and the various options that are built in to it.
There are various options given on the camera with regard to flash. One of them is that of auto flash. This option basically depends on the intelligence of the camera and at times, you will feel that the right results are not achieved. While that is acceptable for routine pictures, auto flash should be avoided when taking pictures professionally. You should either have focused light around the subject or should turn on the flash. It is entirely up to you as to how you want to set the auto flash feature on your digital camera. If your camera is old then you will have to manually adjust these settings accordingly.
Diffuser basically controls the flash. Too much flash can spoil an image. What the diffuser does is that it keeps in check the intensity of the flash. The images that you will see after this will be softer on the eyes.
Most cameras these days have factory installed diffusers, but in some versions, you have to attach a diffuser.
If you need any further assistance in terms of learning how to use the flash on your camera then you can go online and you will find some excellent video tutorials that show each step depending on the various camera models. Check some of these video tutorials and you will get a better understanding of the different techniques involved with setting the flash of your camera properly.