like most things (culture, slang, idioms...) you'll probably just have to learn them from talking to croatian people, watching films, listening to music, reading books...

you could try to learn some from a list or dictionary (here's a website with croatian slang: http://www.zargonaut.com/ ) but there's way too many words and idioms to learn, and to tell you the truth, although i am a well read native speaker - most of them listed on the webpage i mentioned - i don't even know myself.

so, your best bet is just immerging yourself into croatian language and culture and picking things up as they come by.