I prefer to use official docs, if available, e.g. --help, man pages, GNU info, /usr/share/doc, official website. The official documentation is usually more correct and more comprehensive than other resources, though sometimes a bit technical. Other than that, the arch wiki (and I'm using Debian) is a good place to look.

If I'm seeing an error, I find it's most useful to collect as much information as possible. If it's a GUI program, you'll need to start it in a terminal, it'll likely spam errors and warnings to stderr. Many programs have -v or --verbose or --debug options, or a debug mode that can be enabled in the configuration file. Look at the logs if there are any (usually through journalctl these days). It's sometimes already obvious what the problem is just from the log/stderr output, but if not then just paste it into Google.

Also, official bugtrackers are good. When I encounter a bug in a package, I go to the Debian bugtracker first, and almost always someone has already reported it, and left useful information there.