If you still don't get it, pull the lever on the handle bar and watch the motion of the cable, at the rear wheel. That's what you want to adjust. The cable should go thru something attached to the lever on your rear wheel. That thing has a screw or bolt on it, and that's what you loosen and pull the slack on the cable from. pull or move the arm on the rear wheel forward and hold it, then pull the cable thru to tighten it.
If this dosn't help your pads may be worn out. You can check this by using the view holes on the side of your brakes--They have little rubber plugs to keep them covered. If you look thru the holes, you should be able to see some brake material.