First off, CONGRATULATIONS!!!! Its an awesome day when you get your first DSLR! Second off, I'm a noob so take my critique more lightly than others around here. Some of these people have been photographing longer than I've been alive

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I like the first one cause it feels fun even with the facial expression (and will be great for memories), but it looks like her face is kinda overexposed. You might be able to fix this in a Photoshop-like program. I don't know how, but I'm sure you could somehow.
The second one is a really neat photo too (that bird has some awesome eyes), but the background looks properly exposed making him underexposed. Like maybe the background is in the sunlight and he's in the shade? Once again, I don't know if it could be fixed or not... probably so.
Third one is cool. With my beginner's opinion, I would maybe try cropping off a bit from the right first. See if that doesn't make you happy. But remember its mostly about what you think is cool looking. If you try and take photographs that make everyone else happy it will quickly become a chore and lose its fun.
Overall, I'd say to become acquainted with your camera's metering. If you already know about it, then my mistake. But if not, remember the camera's instruction book IS YOUR FRIEND.

I seriously hate reading instructions, but a camera's book is basically a small "how-to manual" that really helps. Also, if that's not enough, they sell lots of books at the book store specifically for Rebel XSs. They have one for Dummies, and those books are usually pretty good (even if you aren't a dummy).
Edited to add: I misunderstood you. If you SOLD your XS and are getting a 40D you probably have learned the basic principles of metering, focus, aperture, shutter speed, and ISO. But they still sell some (more advanced, serious) books for 40Ds. I even still saw one in there for a 20D and a D70!