The other answers to this question are pretty good, but just want to add a little from my own experience. Firstly, don't try to memorise the tables of noun declinations by heart. I did this, and it still doesn't really help in real situations because you have to conjure up an image of the table in you head and select the appropriate form - which just isn't possible in the timescales of a real conversation. The best way to learn all of the declinations is just to try speaking with a native speaker and choose an ending at random. Or just say it in nominative. Chances are you'll still be understood, and if you're lucky they'll correct you. After being corrected four or five times, your brain automatically starts making the link, and it will be more intuitive and natural than attempting to memorise tables. However, it is important that you understand the grammatical functions of the cases (why we use genitive in certain scenarios) and what each of the cases endings look like - so I'm not saying to stop studying grammar, just not to get bogged down in trying to memorise details. It is possible to figure out the appropriate case to answer a question. The clue is in how the "co? kto?" questions decline, as each of the variants (kogo, czym, czemu) resembles the masculine adjective declination form. So for example the question: "Czym interesujesz się?" could be answered by "polskim filmem", where the form of the adjective polski ends with -im/-ym to match the form of co (czym). This doesn't help you much with muzyką, because it's a feminine noun and therefore its adjectives decline in a completely different way that doesn't match the czym question.

The University of Pittsburgh materials are generally very good at explaining grammar rules. For this question, it's worth referring to page 21 of their "grammar in a nutshell" document. http://polish.slavic.pitt.edu/firstyear/nutshell.pdf As far as I can tell, they're basically exceptions to the general rule and the only way to remember which non-animate masculine nouns have their genitive ending in -a or -u is to try one and see if you get corrected or not. Once you've been corrected a few times, the pattern starts to sink in and it becomes second nature.

episode 3 - italki

I'm I've been here for a few days and now I know something/a little more about this site. But I'm not sure if the information is fully valid. First, as a non-paid member of this community I have only one way to practice my english here and this it: the notepad and this writing. The second thing I can do here is correcting notes taken by people learning polish, which is the thing I like. Probably because I feel better in polish than in english ;) Hopefully this will change somewhere/sometime in the future.