Good job, Pacu.
I notice that the subject is usually more or less centered in each of your pictures. You might try offcentering them some (about a third of the way from the top, bottom, or side is typically considered an ideal amount, hence the "rule of thirds" which isn't really a rule, but a good guideline). Also, if the horizon cuts horizontally across the center of the frame, as it does in the cityscape, you can try cropping it so it's about a third of the way from the top or bottom. In the cityscape with the water in the foreground, the water is empty space, or "negative space" I believe it's called. If you crop the bottom, the image could look more balanced.
A search for "photgraphy composition" will turn up lots of techniques.
Oh, and one great technique I've found to make a white building to stand out against the sky is to use a polarizer, which darkens the sky. You have to be careful however. I've more than once used it for that and wound up removing the reflection from the water, leaving it an unattractive muddy color. Still, a polarizer is a great investment! I love mine.
Hope this helps, and keep it up! I look forward to more photos from you!