Great start! You've got a good eye and it will only get better from here on in. I like number 1 and 4. Nice subject isolation by shooting against the blue sky. My only words of advice would be to ask yourself "what is my subject?".
For example, the viewers eye tends to lead to the brightest part of the image. So with the seagull and the camper van, I was unsure what was the subject. The camper van is big and bright and fairly in focus, but the bird competes for the viewers eye or vice versa. Try using a shallower depth of field (DOF), which means (assuming you are using the 18-55 3.5-5.6 at 55mm) opening up the aperture to f/5.6 instead of f/9. That way the seagull will remain in focus and the scenery behind it will be a wee bit more out of focus, which then tells the viewer you were indeed aiming at the seagull.
Numbers 3 and 5 could do with the same. In number 3, the seagull's butt blends in with the truck, and in number 5 it blends in with the airplane. I think I get what you were trying to do in number 5 by showing natural and man powered flight. Just a few more seconds and the plane and bird would have seperated and a more dramatic compostion.
Great start as I said, and the beauty of digital is that learning is free! Congrats on your D40 purchase!
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Ancient Canadian proverb: 'Don't eat yellow snow.'
D300 and assortment from FE to 200mm