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.

Have you Slavs envisaged why you more often than not speak ill of each other?

Only in the 17th century did the Germans unify their languages into one.

Czechs, Slovakians and Poles together fought against the Teutonic Orders in Grunwald and against the anti-Fuss German nobles. The First Pan-Slavist Conference took place first in Chechia in the 19th century, where Checzs was desparately trying to resurrect their ethnic language. During the WW2 Poland and Czechoslovakia agreed in London to form an unified a large West Slavic country, which came to nothing due to Stalin's strong objection. Later, Soviets fed Czechs and on the other hand exploited Poles so that the two nations hated each other. And now you see what.

Pathetic Slavs....