Hey,
Firstly i would suggest that you try and find the ducklings family, if you can then release it back with them. Otherwise you are going to have to do certain things...
Make sure that the duckling doesn't get wet. Many ducklings can die if they get a chill through being wet if they don't have their mothers to keep them warm. Adding a heat lamp to the cage will benefit the duckling. But be careful not to go to the other extreme and over heat it. Give the duckling a shallow water bowl the depth of its head (so that it can keep it's nostrils and eyes clean).
As for food, it depends on the ducklings age. If it's still got it's down/fluff then it's generally under 4 weeks old and you can feed it chickcrumb as this is a good nutritional food for ducklings. Otherwise if it's starting to, or has got it's feathers then feed it a diet of mixed corn.
Other foods that you could feed it, but not too often is lettuce, bread, grass, insects (snails, worms), weetabix and dandelions. Most of these foods are a ducks natural diet anyway.
Give the duckling plenty of space and a chance to exercise (preferably outside in good weather), but always keep an eye on it. Ducks are social birds and benefit greatly from company of their own kind, if you want to keep it then it may be best to buy it a friend. Or, you could release back into the wild when it's older.
If it's not possible for you to look after it then you could try asking for help reguaring rehoming from Veterinary Surgeries, Poultry clubs/organisations and zoo's/parks.
If you plan on keeping it then there are many good books available on duck care such as...
* Ducks and Geese - A guide to management. By Tom Bartlett
ISBN 1 85223 650 7
* The Domestic Duck. By Chris and Mike Ashton
ISBN 1 86126 402 X
Both of these books covers everything that you would need to know.
I hope this has helped abit, if you need anymore help then just ask 
Mel X