How to use flash on a camera properly?

Here's a picture of my attempt at bohek photography of a dead flower.

It was on 105mm at 1/10s, F/5.8, ISO 400, EV -2.0, Flash EV -1.0
I'm new at photography and I was wonder how could you get the background brighter when using flash on a Nikon D90. It seems like the flash makes the object near the camera to appear really bright compared to the background.

The to properly expose a scene you've got the aperture and shutter. When you add a flash to the mixture, the aperture control the exposure for the flash, while the shutter is used to control the exposure for the ambient light. The reason is that the flash fires so quickly that a change in shutter speed can't have an affect on the light emitted by the flash.
You should consider also using warming gels, which are just pieces of plastic, to warm up the light. Just tape it over the flash head and you've got a nice warm-toned light source that will simulate the setting sun.

Light follows the inverse square law. The closer the flash is to the subject the more rapid the light 'fall off' behind the subject.