For night landscapes, a tripod is the best first purchase before another lens. It lets you use longer exposures at low ISO for cleaner images.

For settings, use Aperture Priority or Manual mode. For general landscapes and blue hour scenes, start around f/8 to f/11 with a low ISO, and let the longer shutter speed happen on the tripod.

For stars, use the widest/brightest aperture your lens allows and a higher ISO. A wide lens is usually best for night sky work; something 24mm or wider is a good starting point, and a fast lens around f/2.8 or brighter helps.

A cheap 50mm f/1.8 can be a good low-cost lens, but it’s not usually the best choice for wide night-sky scenes because it’s much narrower. If your goal is stars over landscapes, a wide lens is generally more useful than a 50mm.

So: start with a tripod, learn Manual or Aperture Priority, use low ISO and f/8–f/11 for landscapes, and use a wide, fast lens with a higher ISO for stars.