This sketch is alright but doesn't seem to be quite finished. Look at the same sketch below to see how a simple border can help.
...but a border makes all the difference!
By adding a border the picture makes more sense. The area of the sky is defined as a lovely negative shape, and the walls of the buildings don't just drift off into nowhere. The whole sketch looks so much more complete.
Some more examples
These three small pencil sketches taken from the same page of my sketchbook show how effective the border can be. Just imagine the same drawings without the borders.
Your drawings will also look better if you draw right up to the edge of the paper. This will result in a better picture than a small sketch floating in the middle of a large white expanse of empty paper.
Try starting your drawing at one edge of the paper and work your way across to the other edge. See what difference that makes to your composition.