>>3685904rule of thumb,
aperture -> as wide as you can
shutterspeed -> as long as you can without getting star trail.
iso untracked -> 1600 to 3600
iso tracked -> 400 - 1600 or as long as you can without blowing out your highlights.
don't worry about "iso tracked" by the time you get there you'll know what you're doing.
to focus on the sky, go in live-view, zoom in 10x on a star and manual focus until it's the smallest it can be.
if you want to talk milky-way shots, you combine 2 photos, one of your sky and one of your foreground.
your sky is most likely going to be a number of exposures stacked together using a software specially made for stacking astrophotography. you stack your sky images to get a longer exposure without having stars trailing. example: 20x 5sec = 100sec exposure stacked.
Hope this helped.
