Consider this, the Aruba resort I stayed at:

So how do I decide on the settings for a shot like this?

First, of course I need a tripod.

1. I want the trails. To get these, I need a long exposure time, of 10 seconds.
2. To get this, I need a low ISO, so I start at 100.
3. Then, with exposure time and ISO given, I figure out the aperture I need. Which was f/16.
4. That’s good – because the “starburst” effect is due to the small aperture of f/16.
5. If I had needed a smaller aperture than f/16 (say, f/45), I would have had to increase ISO. If I had needed a large aperture (say, f/4), I would have had to wait until it was darker, or I would have had to use a neutral density filter.

Simple. Right?