Language / Using Polish declensions.. [14]

Basically I'm an English student heading to Poland on the erasmus scheme this year, I've been learning Polish since September and have learned a lot of it very fast within these nine months to the point where I can hold conversations with my Polish friends in Polish, all of which are amazed with my pronunciations and all the words I know.

However my problem is, that I really don't know how and when to use declensions other than locative and nominative nouns and adjectives. I can decline words no problem if someone said decline this word under the dative grammar etc, but what I'm struggling to do, is when to use them as I don't understand the ruling behind it, I've been through the internet and this forum reading as much documents etc on declensions, but I still don't understand it.

Any help will be massively appreciated as this is pretty much the only issue I have.
Dzięki :)