Water
You must buy your hamster a rodent water bottle. These water bottles usually connect to wire cages (from the inside or outside) or hook onto tank cages. It is best to read the piece of packaging that comes on the water bottle to make sure it is non-toxic and resistant to ultraviolet rays. Make sure the water bottle you buy has a ballpoint steel dispenser tube that resists dripping and discourages chewing. Fresh water is extremely important for your hamster. You must dump out old water, rinse bottle (inside and out), fill with fresh, clean water, and dry off outside of bottle every day. Some water bottles I recommend are (pictured from left to right) All Living Things Small Animal Aquarium Water Bottle (has a bracket that sticks to aquarium walls and bottle is removable for filling and cleaning, but is only available at PETsMART) or L/M Animal Farms Water Wells Water Bottle for Hamsters, Gerbils, and Mice (hooks to outside of wire cage and is resistant to ultraviolet rays). If you buy a water bottle that doesn't hook onto or connect to a tank cage, and you have a tank cage, you can buy Super Pet Suction-Cup Bracket Attachments (for securing water bottles and feeders to flat sided plastic/glass aquariums, laminated enclosures, and more). Also, for easy reach, longer sipper tubes can be purchased.
*Health Note* When washing out your hamster's water bottle, make sure you clean out the piece with the sipper tube on it very well. If you don't, it might get clogged with small particles and harbor bacteria and will make your hamster sick.