Hi Marija--if you only have one brake, the one which is working should be on the front! You probably want to take it to a bike shop to get it repaired.
Or, you can try posting a thread in the Bicycle Mechanics forum here. Take some good photos so that we can see what the problem is, and we may be able to help you out.
Oh, and don't worry--there are a lot of people who think that this is a motorbike forum
.
East Hill