One of the most important things to consider when sketching things are placement and proportion. With an awkward placement of even a single feature, things will look weird, and other things will be skewed along with it when you try to compensate. Same goes with proportions. If you don't already do so, you could try starting off sketching things by observing the form of the whole thing, scribbling down its general shape, and then adding details on top of it (this is why it is so crucial to draw light at first!) Good start! Keep on practicing.