Hello, mitchel and welcome! Oh, and congratulations on the baby.
Spot metering is used for when your subject is lighted very differently than its surroundings. For example, you want to take a photo of a person who is in the shadow, and the background is a well-lit white wall: if you use area metering, the person will probably look to dark.
I rarely use spot metering. But you may need it, depending on how photo opportunities present themselves to you.
Are you sure the Canon XTi / 400D doesn't have spot metering? Maybe it calls it by other name?
Anyway, read this
to learn the basics of how exposure works.
There is also a great tutorial by Nikon
(with most of its information not specific to Nikon users) that shows some photo techniques.
Now, if Camera Labs had tutorials... (hint, hint)