New cables do stretch initially, then they settle down. After a while, brake blocks/pads wear and this requires further adjustment. That's why proper bike shops usually include a 1st service after 6 weeks with any new bike - to retension the stretched brake and gear cables, plus safety-check wheel nuts etc.
You don't say what type of brakes they are, but in (almost) any case there should be a cable adjuster at one end or the other (or both ends). You would just need to twist the cable adjuster with your fingers until the slack gets taken up and the brakes work again. If the inner cables are so loose that all the available adjustment is used up, then you'll want to wind the cable adjusters to their middle point (or even to their loosest) and then undo the cable clamp nut at the 'wheel' end and pull some of the loose inner cable through. This will probably need a small spanner or maybe Allen key, plus a pair of pliers to pull the cable with. Then tighten the clamp again and go back and adjust the cable adjusters out again until it all works.
If these instructions don't make sense, it might be because I'm trying to guess what type of brakes you have – cantilevers, side-pull or V-brakes. Or even cable-operated disc brakes. Maybe post a couple of pictures of the brake lever end and the 'wheel' end so we can see.
If you're not sure about working on brakes maybe visit your local (proper*) bike shop. (* i.e. if you bought it from Tesco or Amazon, maybe find an independent LBS). They would probably (if they're any good) do it there and then (5 minute job for a bike mechanic), charge a pittance and show you how to do it yourself next time.
EDIT: just seen in your gallery that you have a DBM05, which has a cable-operated disc up front and a V-brake at the back. I think in either case you will need a 5mm (?) Allen key to loosen the cable clamp. As you said back brake, I would say set the cable adjuster to its mid point, squeeze the V until it applies the brakes, pulling the cable through as you do so, then re-tighten the cable clamp. Adjust with the cable adjuster where the outer cable ends until the wheel can rotate without the brake rubbing at all, but obviously make sure it is tight enough to stop the wheel rotating well before the lever gets near the bar.
