By alien I mean different, so this could mean the future or more literal aliens.
Having an alien setting feel alien quickly is something you likely will have to do, never forget the first page of your book is by far the most important.
The first thing to keep in mind is that alien should not mean incomprehensible, it means different. Don’t go overboard too fast, always make sure there is enough understandable stuff for people to understand and relate.
You can start by changing simple things just enough to be noticed. For example the units of measurements, or the color or shape of things. If you describe a car as being spherical or say that your three plinks late for a meeting you establish that you are not in 21th century earth.
Another thing that you can try is to casually mention one of the big differences between them and us. You don’t have to describe it, just casually mention something that makes them different.
For the start of the book do that, mix the familiar with the new until the new is relatable.