cases

I'm working on learning case endings, and I'm stuck on the accusative case. I understand that masculine animate nouns don't change. The same applies to neuter nouns. My problem lies with feminine nouns, I think. Feminine nominative nouns end with -a and -я. In the accusative case, they end with -y, -ю, -ь, or -ию, right? I just don't know when to attach which ending. For instance, книга becomes книгy. My book told me that one, so I'm confident about it. Same thing with неделя - it becomes неделю. Does that mean that я always becomes ю, and a always becomes y? I haven't found any hard and fast rules for this, so I'm still a bit unclear on it.

This is my second time learning a language with cases, admittedly, so I shouldn't be having this much of a problem with it, but it's been ages since I've even thought about Latin.