The best species for beginners is the syrian hamster(aka teddy bears). They can reach up to 8 inches long, but average is 6-7. Syrian Hamsters are solitary and must live alone.
Males in general are calmer, but you will get the occasional female that is mellow too. Females go into heat every 4 days and have a bit of any odor.
Food to feed is hamster food. The best food you can get on the pet market is called Hazel hamster. Most others contain loads of preservatives and random junk that is not needed.

Hamsters need large cages with lots of room to play (I reccomend aquariums or modified storage bins). Those commercial cages with all the pretty colours really are a waste of money. They fall apart, are to small, providing the hamster no room to play, and are just plain exspensive. They need a solid running wheel that is big enough for when they are fully grown.
As for bedding, pine and cedar are toxic. I would recomend Carefresh, KAYTEE Soft Sorbent, or Aspen shavings.
Average like span is about 1.5-3 years.

Oh... and lettuce (iceburg is very bad and can cause liver failure. Other lettuces, like romaine need to be fed in moderation)

In general, hamsters make great pets and are very easy to care for.

I will reccomend you check out these links:
http://www.petwebsite.com/hamsters.asp
http://www.geocities.com/calhamassoc/Newsletter.html