If possible, get it removed and a normal meter put in, you can pay upto 20% more for your electricity buying it this way. I had a card meter for Gas and Electric when I bought a house 5 years ago and kept them both as I thought they would help me budget better, not realising how much more expensive they were!
Your energy supplier should send you a card (or a key) for use with the meter, which you then top-up with credit at PayPoint or PayZone outlets - usually newsagents, corner shops, petrol stations etc. Again, your energy supplier can tell you where the nearest ones to you are.
Once your credit runs out, the lights go off!! You usually get �5 'emergency credit' which you can use if your money runs out but if you use that up as well, you will be sitting in the dark until your nearest shop opens so you can top-up!