Whichever type of cage you choose to get, make sure that it is big enough and cleaned out regularly. The hamster's toilet area should be cleaned out daily - this is easiest with a special litter tray - and the whole cage cleaned every week.
Hamsters can squeeze through very small gaps so only choose a cage that is designed for them as they may fit through the bars of other types of cage. Dwarf hamsters may even fit through the bars of standard hamster cages so a solid plastic cage may be best for them.
Hamsters love to climb and dig so provide them with plenty of toys and activities in their cage. Many hamster cages for sale come with several different accessories so you should not have to buy all the items separately.
An exercise wheel is essential. Do not use one with open rungs as hamsters can get their legs caught between the rungs. If your cage comes with an open-runged wheel, buy a new solid one and use that. Provide lots of interesting items for the hamster to investigate. Toilet roll tubes and cardboard boxes are a favourite - they will be chewed up so be prepared to replace them as necessary! A pesticide-free branch should be provided for gnawing - apple wood is best. Supervised out-of-cage exercise can be given using an exercise ball.
A water bottle should be available at all times. Water bowls are not advisable as they can get very messy.
A food bowl is not essential as hamsters like foraging for their food (scattering it around the cage encourages this). If you do want a bowl, choose a very small one as larger bowls encourage over-feeding.
The hamster should have somewhere enclosed to sleep. A plastic house or suitable-sized cardboard box is ideal. This should be filled with special bedding material. The rest of the cage should be filled with wood shavings (not sawdust).