Language / Having a really hard time with Polish cases [59]

I generally do really well in my Polish course, but lately I am having a real hard time trying to determine when to use which case. I understand how to change the case endings, however when to use them is becoming a nightmare. I have just started my third Polish course and it is becoming increasingly difficult because I do not understand how to do this. We are writing stories and what not and my cases are all over the place. If anyone can give me an explanation or a thorough reference book I would greatly appreciate it.

Thanks :)