How do I blacken the background while taking a picture? (Canon Rebel T2i)?

I'm taking a picture of an outdoor wall lantern at night. I want it so that the lamp is in focus while the background is black, so that it looks like the lamp is almost floating. I set it to a high shutter speed in hopes of making it darker, and when I press the "shoot" on halfway (autofocusing it), the background will be dark the way I want it. But when I take the picture, the light cast onto the objects by the lantern appear again. Please tell me how I can adjust my settings (i have a canon rebel t2i). Thanks!!