Know your flash's range. This is a very important technical aspect, that is mastered by experience.
Digital cameras support many photo shooting modes - from automatic point and shoot to manual where the camera does nothing for you. One of those modes is Aperture Priority. Aperture priority is useful in many situations and deserves a short explanation of what it is and how to use it.
You set the Aperture priority mode by using the camera menus on its LCD in which case you need to browse and choose Aperture priority from the menus or by rotating a modes dial in which case you choose Aperture priority by its symbolic icon (in most cases Aperture priority is symbolized by a capital 'A').
In Aperture priority mode you manually set the aperture value. The camera takes care of everything else - for example it sets the optimal shutter speed for the aperture you chose. There are physical limitations and not every aperture value that you choose can be matched by other settings that will result in a good photo. The camera will let you know by flashing a green LED or in some other way (check its manual for more details) if it found the optimal settings that work with your chosen aperture value.
So why bother setting the aperture value manually when you can have the camera set it for you? One good reason is to control the depth of field. Depth of field is defined as the range in which the photo is in focus. For example an infinite depth of field means that the photo is in focus from a certain distance from the camera and up to infinity. A narrow (also known as shallow) depth of field on the other hand means that the photo is in focus only from a certain distance from the camera to another distance which is not further away (or in other words the photo is in focus just around a certain object that is being photographed). The rest of the objects in the photo are out of focus appearing as blurred objects.