Welcome to YorkieTalk! You can start off by reading the Frequently Asked Questions thread if you haven't already:
http://www.yorkietalk.com/forums/showthread.php?t=6016
Yorkies are very small, the standard size is 4 to 7 pounds generally, so it doesn't cost too much to feed them. However, if you start spoiling them like Yorkie parents around here do, then it can get a bit expensive with clothes, jewelry, toys, doggy beds and the like. A Yorkie's temperament will vary, but overall most are pretty good. The can be housetrained fairly easily, it takes between a few weeks to six months for most, it just varies on what style of training you utilize.
That is very wise of you to do your research before getting a dog. Good luck in your search!
YorkieTalk also has a sister site for Chihuahuas at
http://www.chihuahuatalk.com