As I'd explained in the first step of creating an alien race, good science fiction always has human characters. Not all humans have to originate from Earth, though. In my novel, Web of Deceit, only one of the human characters is from Earth. The rest of them, including the main character, are from a human-inhabited planet called Dheelia. In Joe's novel, The Quartz Blades, his human characters are native to a planet called C4-783.
This is generally accepted as normal by SF readers today. But…a quick aside on this (because it's not really the main focus of this post, but it's worth mentioning), if you're going to have other human-inhabited planets in your made-up universe (other than Earth, that is), it wouldn't hurt to include a quick mention near the beginning of the story of how those humans got to be there…perhaps they'd always been there (in the same sense that we've "always" been on Earth), or perhaps, as is the case with my novel, humans from Earth had colonized the planet once they'd discovered it, or whatever your reason happens to be. This may seem insignificant, but trust me, your readers will be curious. Even if it's just a single sentence about the topic, it will make a difference.
All right, now that we've got that out of the way, on to the aliens.
2) What planet is your alien from?
Every sentient being has to live somewhere, and the most likely choice is a planet. The definition of alien, according to Webster's, is: (adj, first definition) foreign; (n, third definition) a hypothetical being from outer space. Since we haven't discovered true aliens among the stars yet, everything we write about is quite hypothetical, which means you could, in theory, have an alien race that lives on a comet. But where's the fun in that? Aside from being able to surf its life away through the galaxy, there's not much creativity involved.
However, creating an entirely new world--a whole planet--can really bring out your imagination. Again, though, the alien has to fit the planet you force it to live on.
So you've already decided on a good or bad alien, in relation to your human characters. And from that, you've most likely come up with some defining physical traits. Those characteristics are what you need to keep in mind when creating the aliens' home. Here are a few things to consider:
Atmospheric conditions: How does the alien breathe? Does it need oxygen, like we do, or can it breathe other gases? Does it even breathe at all (in the way that we think of breathing)? Hmm…
Land to water ratio and elemental details: Earth is (unofficial stats here) about 75% water, and, not surprisingly, our bodies are about 65% water. What elements, minerals, chemicals, etc, are important to the vital functions of your aliens' bodies? This would include the food they eat, or whatever it is they use to gain the needed energy for survival.
Planetary position and details of its star system: This is more important than some (who aren't nerds like me) may think. The reason Earth is the only planet in our solar system with vast amounts of plant and animal life is because of its distance from the sun AND its axial tilt. A while back, I had posted a snippet on the Writer's Digest Sci-Fi/Fantasy forum about the planet Venus. Interesting planet, to say the least. If you ever have time to research it, I highly recommend doing so, for the sole reason that it proves my point here. There are very good reasons why humans (and, I would think, most living things) don't exist on Venus, even though it is very similar to Earth in its size. Venus has been termed "Earth's twin", but it's so, so NOT that at all.
Every planet has to be part of a star system (our sun is star, in case anyone forgot). They are what make up our galaxy as we know it, and no doubt, the whole universe. There are some things that you just can't make up, even in fiction, especially if you want to be taken seriously.
Another aside: Please remember, unless you're writing Star Wars fan fic, it takes longer than ten minutes to travel between planets.
All these aspects of what makes up your alien planet will require a good amount of research, unless you're already a professional in any field of science. But don't dread the research. Embrace it. Once you get going, it really doesn't feel like work. Just keep reminding yourself why you're doing it (uh…that would be…YOU'VE GOT AN AWESOME STORY TO WRITE…yeah, that's it), and you'll be fine.