Not a language expert, but I've had some success gaining general fluency in one (non-native) language, and at least a smattering of a half-dozen others.
Basic language courses (e.g., Berlitz, Duolingo, Rosetta Stone, etc.) can be useful in giving a basic grounding.
Many countries have both native-language (that is, source country, not audience) broadcasts and podcasts and language-specific instruction. I'm more familiar with German-language resources here (especially the Goethe Institute and Deutsche Welle), which are excellent, but have had success in other languages (though not specifically Polish).
Children's stories are an excellent basic language-learning tool. The grammar, syntax, and vocabulary are basic, but they do cover the fundamentals, and provide a good basis for further skills development.
Once you've gained basic fluency, one podcast series I've recently discovered is the Easy Languages family of podcasts ... which includes an Easy Polish channel:
Easy Languages: <https://www.easy-languages.org/>
Easy Polish: <https://www.easypolish.org/>
Having your current foreign language playing constantly is an excellent way to simply become attuned to it. Increasingly I find I'm learning to recognise specific words and terms long before I know what they mean specifically, though I can often work that out from context.
Easy Languages also has an overview podcast which has 22 episodes mostly focused on the process of learning additional languages, several of which should be of specific interest to you: <https://www.easy-languages.org/podcast/episodes>
More generally: use your podcast app to find and subscribe to podcasts which are either specifically language instruction, or simply target-language, in your language of interest.
For Poland specifically, using ListenNotes <https://www.listennotes.com>:
- There's the Polish national broadcaster Polskie Radio, which has at least six podcasts.
- Searching for "children's stories" in Polish, "opowiadania dla dzieci", turns up a couple of results.
- "polski angielski" (polish english) gives several thousand results.
You might also want to supplement with a text on introductory Polish (for Dutch or English speakers), or a local adult education / community college (or equivalent) course.
Others:
"The Best Resources for Learning Polish" <https://www.clozemaster.com/blog/best-resources-for-learning...>
"Polish Language Classes: 48 Resources and Lessons to Learn Polish Free Online" <https://www.fluentin3months.com/learn-polish-free/>
