OK, well, I didn't personally study Just War, but there's a good Edexcel textbook for Ethics called 'Edexcel AS Religious Studies' which gives good basic information - the 'Question and Answer Guide' is also pretty decent too. They're both available from Edexcel or from Amazon.
As for how to answer questions - the a) parts are usually pretty easy as far as I can remember, because they ask you to describe key features or something like that. So you need to know, obviously, the key features of arguments and theories, philosophers that back these theories up, examples, analogies, etc. There's always something about evaluating strengths and weaknesses as well, so you need to be able to evaluate. These questions basically require a VERY short intro (like, a sentence stating the argument), then the strengths and weaknesses of the argument - which is stronger, etc. The best way to do this is to, for example, say a strength, then counter-argue it with a weakness, then counter-argue the weakness with a strength, etc etc.
Does that make sense? If you've got an essay you'd like me to look over, I'm quite happy to, just PM me.