I expect it is a challenge every Science Fiction author faces. Let's face it, if you are writing about interstellar travel, you are, at some point, going to have to meet alien life forms. It is extremely unlikely they/it will look anything like we do. Why? Put simply, because "life" tends to adapt to a range of things such as the environment it inhabits, the available food sources, the kind of atmosphere it lives in (and water is an atmosphere ...), the gravitational field of the planet, and even the radiation it must live with emitted by the local sun. If you think about it, you rapidly realise that this raises a number of challenges when you expect humans to interact with it.
Okay, as an author, you are essentially "creating" the kind of lifeforms your story demands, so most authors in this genre tend to create hominid types, and for convenience, we give them "languages" that can be translated into "human speak". Well, if we didn't, we'd have a pretty tricky situation in the storyline, but that, in itself creates a small difficulty. Language tends to be need/environment specific, thus idiomatic expressions often don't translate between languages from different cultures - and sometimes even lose meaning in the same language as used in different parts of the world. It is famously said that the Inuit peoples have something like forty words for "snow" and I am told that if you ask someone from the tropics what word they have for it, you will get a blank stare - they simply don't have a word for something that doesn't happen in their environment.
Researchers have now established that many animals communicate very effectively, and can convey quite complex messages by combinations of sounds, expressions or gestures, so language is not unique, and is not confined to words as we know it. Dolphins, Orcas and Whales all convey complex information by means of whistles and other sounds, some of which are too high or too low for us to hear. Bees can also communicate exact directions to sources of food by means of complex movements and perhaps scents we can't detect. The list goes on and on, and that is just communication!
Body shape and structure is another complex issue. On Earth life has tended to develop as either land dwelling or marine, but there are branches of both best described as amphibious. Fish with lungs and fins adapted allow movement on land, and air breathing animals adapted to live in water but capable of coming ashore and living there. Where humanity evolved in the niche between water and mountain peak, other species evolved to live in rather specific habitats everywhere in between that range, and so it may be with any aliens we encounter when we eventually do begin to explore the vastness of space.
Gravity on Earth being what it is, we (and many other species) have evolved a bone skeletal structure to support or organs and which shapes us. We are adapted exactly to function in an upright stance, on land, within the gravitational forces of our planet and with an oxygen content in the atmosphere of between 18 and 22% Below that level we need life support systems, and above that will result in oxygen poisoning and death if sustained. Others may have developed in atmospheres with less oxygen (If you go above 25% oxygen the planet is likely to be extremely combustible!), or even to breath something else. It is fairly certain that prolonged voyages into space (and here I'm assuming we would do so with some form of artificial gravity) will certainly cause evolutionary changes to the travellers over a prolonged period. Some argue that these would be obvious by the third or fourth generations and I can certainly see the rational there. After all, our current environment on our home planet both shapes and restricts us, so someone removed from it, and placed in an environment that has, perhaps, a lower gravitational pull, lower oxygen levels and a different source of food, may begin adapting to that.
They would probably grow taller, their bones become lighter and possibly less rigid. Hair might not be retained, longer, more flexible fingers could be an advantage ... The list is endless, and, of course, pure speculation at this point in time.
So, as an author, how do I want my aliens? As part of the story they need to be, for me, within the bounds of the realistic. They generally need to be air breathers otherwise the story gets into all manner of complications as you end up with half the "cast" in environment suits of some kind and the need to keep changing through airlocks. I choose to make them hominid if they are going to talk/work with humans. It could get tricky again if your alien is a squid-like creature since they are perfectly adapted for a completely different environment to that for humans. If you don't want that interaction of course, you can get a lot more creative. That is how, in Harry Heron: No Quarter, I set part of the story on a warm and damp planet with the top predator a sort of walking tree, and other creatures that were giant versions of things like the insect eating plants one finds in parts of the tropics. Other creatures resemble large insects and even outsized creatures normally only visible through a microscope. You certainly don't have to look far for inspiration among the plants, animals, aquatic creatures and birds.
So, returning to my starting point, in my Harry Heron series, I've tried to create believable 'aliens' who are able to interact with humans, and others who give a strange planet the sort of diversity you would expect. Have I got it right? You be the judge. You can explore my alien creations in my books.
Harry Heron: Into the Unknown
Harry Heron: No Quarter
Harry Heron: Savage Fugitive
and to find out where Harry and his friends come from, read Harry Heron: Midshipman's Journey to learn why Harry, Ferghal and Danny think and act as they do when confronted by the Lacertains, the Canids and others ...