Use manual focus. For night-sky shots, autofocus usually won’t work reliably, and many lenses don’t focus correctly just by turning to the infinity mark. A good method is to pre-focus on a very distant object in daylight and note that position, or at night use live view (if available on your camera/lens setup) and focus until stars look as sharp as possible.

For the ISS, a wider focal length is usually easier than a long one because the station moves quickly and long lenses make tracking and star trailing more obvious. Community examples used about 28mm on a crop-body camera with 5-second exposures, tripod-mounted.

Keep in mind that even 5 seconds at longer focal lengths (for example 85mm) can start to elongate stars. If you want sharper stars, use shorter exposures and/or a wider lens. If you want the ISS as a streak across the sky, longer exposures are fine.

Expect to experiment: take a few test shots beforehand, and be ready to raise ISO if needed. A faster lens also helps.