The time to use your flash is when the sun is high in the sky. This is when harsh shadows and contrasty light can ruin an exposure. Used correctly, the flash will soften the shadows on your subject without altering the rest of the photo.
Don't let others tell you the pop-up flash isn't effective either. It's actually a tremendously useful tool if you recognize its limits. Due to the small size, the built in flash has an effective working range of about 5-6 feet. It won't reach much further than that. For a really professional look, reduce the flash output to -1.75 in your camera's menu. This creates a beautiful soft quality of light that enhances any subject.