Welcome to the forum. You will get lots of advice here, as you can already see. I would agree with the comment about underexposure and the tilted horizon. Nothing "kills" a good landscape better than an horizon that is not horizontal. My question also concerns the subject of the image. I am unclear in the first one whether the image is the sky. If it is the sky, then the trees hide too much of it. I also think it is a bit underexposed. In the second one, the image is underexposed, although I will assume the subject is the tree with the bit of light on it in the foreground. The third one is not bad as the subject, the sunset, is obvious but again, it looks underexposed to me. As Civchic has suggested, a tripod is almost a necessity when shooting these types of images. If you do use a tripod, then go with a remote shutter release and mirror lockup as this helps to remove any possibility that camera shake (doesn't have to be much) will cause blurring in your image. Hope this helps.
WesternGuy