You should buy a cage for your pet that will be at least 16 x 12 x 10 inches. 
The best temperature for your hamster's habitat is 68 to 72 degrees F. If it gets to below 50 degrees, hamsters go into hibernation, and they rarely survive temperatures below freezing. Also, hamsters easily catch colds if exposed to drafts.

Hamsters are solitary creatures. Commonly they use to fight each other. If your hamsters are fighting, you will notice that once hamster seems more ative and the other seems to act subordinate. Two hamsters should not be housed together is fighting develops.
A gerbil and hamster cannot live together because hamsters are solitary creatures and so the hamster would most likely hurt and eventually kill the gerbil. Gerbils are pretty social animals though, so you could house two of them together. Otherwise you should purchase seperate cages.
If you have bought your hamster a few seconds ago you shouldn't start handling it right away, expecially if it hasn't been handled by humans very much. That could be very stressful and cause your hamster to become sick.
Try to buy your hamster in the evening when it is more active. Earlier in the day, most of the hamsters will be asleep.

The hamster's body should be smooth and well-rounded.
Its coat should be clean, particularly on the underside of its body. There should be no bare patches.
Its ears should be clean on the inside, and if you're buying a young hamster, covered with hair on the outside.
The hamster's eyes should be bright and clean.
You will probably want to buy your hamster when it is between four and seven weeks old because hamsters are easier to tame when they're young.
Male and female hamsters are equally good tempered and agreeable.

You can keep two hamsters in one cage depending on what kind of hamster you want to get. Dzungarian Dwarf hamsters, short dwarf hamsters, and Russian hamsters are all supposed to be pretty social, so you can keep two of one of those kinds together. But it is best if they are litter mates, have been together since birth, and are of the same sex. All other kinds of hamsters need to be kept separately because hamsters are territorial.

Ask the pet dealer what food the hamster has been eating, so you can give it the same. If you want to feed it something different, remember to change to other foods gradually.

The pet store will give you a cardboard box to take your hamster home in. But if your ride home lasts more than two hours the hamster may gnaw its way out. So you can take along a small plastic box. It must close securely and have air holes. Add bedding and a small amount of food to the bottom.