Polish, Czech, Slav... similarities?

Good evening.

Purely as a hobby, i've decided that i'd like to pick up a European language (I already speak a couple Asian languages).
I initially wanted to learn Czech, but i was comparing populations that speak languages in Europe and found that Polish outnumbers it and that it also has a lot in common with slavic languages.

tl;dr:  Anyone who speaks Polish or Czech, or any combination of those Slav languages, are there similarities?  'ppreciate your comments.

>>38
Ancient Proto-Indo-European language is said to have numerous number of consonants.
It means that Poles simply maintain the old PIE tradition today whereas all Slavs created palatalised consonants as well to make things more complicated.