First and foremost, hamsters shouldn't be taken lightly. Many people do this and it's sad. Hamsters are living animals and though they don't require a lot, need proper care and lots of love and attention just like any other animal (or person!)
       First, decide what type of hamster you want; Syrian or Dwarf. Syrians are bigger, and usually more friendly and they make great pets. One thing though, Never put more than one syrian to a cage. They'll fight, often to the death and unless you like this (you are sick!!), don't do it (I have heard of Syrians living together fine but every hamster expert I've talked to has said to never put them together)! Dwarves on the other hand, can be put together and will get along great (but I think they'll usually only get along if they've been together for a while, so I wouldn't get another dwarf for a cage already containing different ones).
       When choosing your special pet go in the evening when hamsters are most active (they're mostly nocturnal). Make sure hammie is "bright-eyed and bushy-tailed!" It should have sleek, clean fur, bright, clear eyes, no bare patches, and clean ears. There should be no sign of sickness (see sickness) and it should seem happy and friendly. Unless you want to spend a while training an aggressive hamster, I would pick a friendly one! Ask a pet store attendant if you can hold the hamsters and see which one seems the best to you. (Actually, I prefer getting hamsters from friends because they're usually nicer and easier to earn trust from so I recommend this).