Avoid making your aliens like humans. One of the most frequent mistakes made by beginning writers is giving their aliens too many human characteristics. Chances are good that beings from another planet will have characteristics that allow them to live on their planets as easily as humans live on Earth. If we were to go to another planet, chances are good we'd have to find a way to adapt on that planet before we could invade it.
Design a language for your aliens. It doesn't need to be complex, but there should be a way for your aliens to communicate with one another in a language of their own. Consider using a language creation tool to help you put together a consistent, realistic language for your aliens (see Resources below).
Create a world from which your alien originates. It doesn't have to be a real planet, but for the sake of being convincing, locate the world in a solar system or a part of outer space we (humans) know nothing or little about. Design your alien world and your aliens together so the characteristics of each support the other. If your alien can't see in the traditional human sense, the world your alien comes from should have environmental characteristics that support this trait in your alien.
Give your aliens personality. Not all aliens will be bad, just like not all humans are bad. Aliens are likely to have personality characteristics that make them something more than just an ugly creature from another planet. In fact, not all aliens need to be ugly. Be creative and create alien characters with as many layers as you give the human characters in your fiction.