That’s too broad so I’ll pretend you asked “What are some sources to learn about authentic Slavic paganism?”

As with all things paganism, beware of claptrap; there’s always someone talking shite with no citations. Watch for American or English people with no qualifications in history, linguistics, or archæology, and no peer-reviewed publications who overuse the words “spiritual” and “nature”.

Everything always comes from primary sources, though primary sources are usually inaccessible to most people. In the Slavic case, there’s quite a bit of archæological evidence (idols like the Zbruch Idol), and texts like the Russian Primary Chronicle and the letopis genre, and oral folk evidence.

• The evidence from historical linguistics (e.g. gods’ names preserved in modern words) was best analysed by Roman Jakobson in Volume VII of his collected works.

• Chapter VIII of Marija Gimbutas’s The Slavs is about religion and is 20 pages long – worth a read regardless of what you think about Marija Gimbutas.

• Linda J. Ivanits’ 1989 book Russian Folk Belief (on annas-archive dot org as an epub) is a grand little general work, covering the bases: Slavic creation myths, witchcraft, all based on interviews/oral tradition.

• Encyclopædia Britannica: https://www.britannica.com/topic/Slavic-religion

• The Mythology of all Races is one of the go-to sources I listed here and has a section on Slavic Mythology: https://archive.org/details/mythologyofall03gray

• Modern Paganism in World Cultures: Comparative Perspectives (on annas-archive dot org): Chapter Six, The Revival of Ukrainian Native Faith, is about 30 pages and covers neopaganism

• Studia Mythologica Slavica seems to be the main journal on the subject, correct me if I’m wrong. Articles in English and Slovenian: https://ojs.zrc-sazu.si/sms – You can search the journal, e.g. a search for the god Triglav

There’s a bunch more if you can speak Polish or Russian.