Anyone who's watched any science fiction TV show or movie has probably noticed how human-like aliens are. Of course, this is because humans play the aliens. Given that the only experience we have with lifeforms comes from our planet, how realistic is it to expect creatures from other worlds to resemble what we know? According to Arik Kershenbaum, the author of The Zoologist's Guide to the Galaxy, even if evolution on exoplanets might not follow the same path it did here, certain traits will still be selected for.
Kershenbaum starts by examining what forms and functions might be common across worlds. In particular, he focuses on features such as movement, senses, and communication. From there, he builds to what we might consider more advanced features such as as intelligence , sociability, language, and more. He even considers whether aliens might be capable of creating artificial intelligence and whether we would think of them as "human."
Kershenbaum may not think much of science fiction, but science fiction writers might be able to get some good ideas about alien creation from this book. The Further Reading section at the end lists many other potentially helpful books. It's always good to improve your scientific knowledge when you're writing science fiction.