What setting are you using? If you're in Auto, then break that habit now. Auto is for when you hand the camera to someone to take a picture of you.
Get the book "Understanding Exposure". Any major bookstore like B&N or Borders should have it in stock. If not, it's available at Amazon. It does a great job of explaining the relationships between Aperture, Shutter Speed, ISO ("film" speed) and white balance. You need to understand how those four elements work together.
There is a whole section on metering as well. Understanding what your camera is seeing is imperative, but get a handle on the other things first.
Oh yeah, don't mess around with the color settings in PS too much if you haven't calibrated your monitor. You spent a lot of money for sophisticated technology in your camera. Trust it.
AMEN AMEN AMEN,
Look, there are five elements of taking an award winning photo. They are: Mental, Linear, Exposure, Emotional, and Factual.
The linear element refers to where the lines are pointing in your photograph. For example, your tree photo, with the deep blue sky behind it. The lines and branches of the tree lead your eyes from the top of the frame to the bottom. Where ones eyes go in the image determines the subject of an individuals focus. If your photo IS NOT A PORTRAIT than you need to think about the linear aspect of your photo. The lines in your photo should be pleasing to the eye. Your tree photo is really good, however the lines lead your eyes from the top of the frame to the bottom and does not resolve on a subject. For example the ground or, the bottom of the tree. The picture leads into an infinite line. This confuses the individual looking at your photo. The lines of your photo should lead to the focal point, or point of interest.
EXPOSURE, EXPOSURE, EXPOSURE!!!!!! An image SHOULD NEVER have blown out areas or hot spots. In many cases of your photos, your sky is white and featureless. If you use a Auto setting on your d40 (which was my first camera, I now have a d300), it will OVERexpose your image. You should only use the Manual setting, even Aperture Priority mode overexposes your image. Also, it is a good idea to buy a polarizer or +4 neutral density filter.
Factual, your image tells a story. Who, what when or where. The best pictures evoke a sense of imagination in the viewer. Always take photos that tell of stories and places.
Emotional, this is self explanatory. What emotion is your picture telling the viewer. If a library, doesn't hire a monotoned individual to read story books to kids, than why would you take a picture, without a sense of emotion. This ties in with the story.
The last and most frustrating element is MENTAL. Why is this frustrating? Because the mental aspect is not created by the photographer, but by the viewer. You could take an absolutely beautiful picture of an individual, and it can contain the best posture, the other four elements, and may be well planned out. Than you can take a crappy picture. The individual will almost always pick the crappy picture because it strikes a mental aspect. For example, the picture has the biggest smile, or the prettiest dress. Not all individuals recognize art and technique, or even elegance. The mental aspect is the hardest to overcome. This is why you can go to a photography contest, and the worst pictures get first place.
As always these rules can be appropriately broken from time to time in unique circumstances. Good luck to you, we all learn over time.
Feel free to critique my photos as well in the other threads.