I have had hamsters for many years, books on hamsters are good to get, or look online for information on hamsters. I have had a look online for you and this website looks very good, and user friendly:
http://www.hamsterhideout.com/feeding.html
I feed mine 'Harry Hamster' which is a staple diet food mix, and also as treats give him apple, carrot, and milk drops / biscuits which you can buy for the petshop. Apple and carrot should be given in small quatities to stop the animal getting the 'runs' (I would give mine 1 thumbnail size chunk piece a day) and the boxed treats say 'only give 2-3 drops an day' i give one a day.
I have heard that the cheap staple food mixes (such as own supermarket make/no frills) are full of rubbish and very unhealthy for your pet. If you think your hamster is currently on a poor quality food it can be changed very slowly by adding the new food in with the old until the hamster eventually eats purely the new food. If it is not introduced slowly the hamster can become very ill.
Hope this helps