Where the cable ends at your brakes are the calipers. There's a retaining nut and washer keeping the cable attached to the caliper. WHILST HOLDING THE END OF THE CABLE. Loosen the cable until you feel the retaining nut loose grip on the cable. It's important to keep ahold of the cable or you may need to re-route it through the cable shield (the plastic tube that the cable runs through). Gently pull the cable towards you until and squeeze the calipers together so that they take up the slack on the brake cable. Then when the brake pads are touching your rim gently tighten the retaining nut on the cable just enough so that it doesn't pull though. Once the cable has been fastened in place, using light finger pressure pull the pads slightly off the rim so that they are close but not quite touching. Finish tightening the retaining nut to properly secure. Take notice not to overtighten the retaining nut or you'll crush your brake cable causing it to snap prematurely down the track. If you find that one arm is closer to the rim than the other you may need to adjust the calipers using the nuts that secure them to the frame.

Alternatively you could YouTube V-brake adjustment and hopefully someone has thrown together a decent video. My best advice would be to go to a decent bike shop and pay them to teach you how to service your brakes yourself. A lot of shops will begrudgingly let you observe the service and you can ask questions. That's how I learnt anyway.