definitely - definitivno
of course - naravno
I don't care - well thats a tricky one. We have many ways to say it. depends on the occation.
simple and kinda polite - ne zanima me.
also simple and less polite - baš me briga!
common and not polite at all - zabole me!
Ooh, also, for a slightly different meaning of "I don't care", you can say, "Sve mi je jedno", which literally means something like "It's all the same/equal to me"
So I suppose it's more like "I don't mind", but I like it because it's simple and common, yet fairly polite at the same time.