A good friend just bought an Expobar Office Control on my recommendation, and asked for some tips to get started. This is about the 4th friend I've directly influenced into getting a serious home espresso machine, so I figure it'll save everyone some time if I put in one place the better reference material I've found over the years. If you find anything else, feel free to leave it in the comments.
Whole Latte Love does a great job of explaining the basics of how to make espresso. (They've been great over the years as a retailer of espresso machines; between my dad and I we've bought more than a handful from them.)
If you learn well by reading, these three articles should be all you need:
The Golden Rule - explains the key aspects of an espresso.
The Ritual of Making Espresso - step-by-step instruction on how to do it right.
The Art of Frothing Milk - hint: it's not about "froth" at all. A quote:
I'll add more in a bit...