No matter which tool you pick, the community will be here to support you.
For darktable, here are the basic steps you can follow:
(Note: it was created using v3.0; with 3.2, things are a bit different, e.g. the grey-point selection slider is normally disabled: it’s recommended to first make sure midtones are properly exposed (exposure slider), and then adjust black and white points in filmic).
There’s a follow-up with a much more difficult photo (darktable 3.0 for dummies: hardcore edition), but you can wait with that one.
Most developers (at least on the darktable mailing lists) will discourage the use of DNG (unless that’s your camera’s native format, or your camera is unsupported).