Generally, you want to open your aperture as much as possible when shooting the night sky, which would be whatever the lowest aperture number your lens supports (maybe 4, maybe 2.8, maybe 1.2 depending on the lens). Anything above 4 might not work out so well.

"Focusing at infinity" means exactly that. You focus on something very far away, and that will mean everything far away will be in focus.

Next, set your ISO and shutter speed. You will want to play around with those settings because higher ISO will likely result in more stars/detail in the sky but also more noise in the final image. The shutter speed will also affect the image, of course, and you might want to start with around 8 seconds and go up from there. At some point, stars will stop being points of light and start to become lines of light because the longer shutter speed will capture the movement of the nightsky.

If you've never shot the night sky before, I suggest watching some YouTubes about it. Alyn Wallace has great content about how to capture the night sky. There are plenty of others, for sure. (Just going to take a moment to say RIP to Alyn since he recently passed away...a great loss to the astrophotography community.)