Hey, I'm going to do English at uni and got A* in Lit GCSE. I was getting Bs till about April, the biggest change came when I began to read A* essays because it meant I knew how I was supposed to think. It's important to be an indeoendent thinker in English, but looking st this is so helpful because it gives you an introduction to the key themes in a text and the general way English is approached. Ask your teacher to give you essays which have got A*s so you can see what you need to do.
In terms of the best notes I could find for OMAM, check this blog
http://englishtutorhome2.blogspot.co.uk/2013/08/all-novels-plays-and-poems.htmlA* notes!
Also, once you've read the text once, use Shmoop for quotes. They'll give you all the key quotes for character, chapter theme etc. Remember to shorten them so they can be embedded and you remember them easily. Reading the text and noting quotes which resonate with you is ideal, but if you're just doing it for an A/A* then this is definitely not necessary.
Also, they can ask you anything, but you have a lot of freedom in what you can say, so you can use ideas from practice essays and the structure should fall into place.
The efficient thing to do is to look at paper questions, then start writing essays on them. Get quotes, use the internet if you have to, and just start attacking it. Key things to keep in mind: GCSE is all about language analysis. Don't reiterate plot, just get a quote which relates to a theme and say what it could link to. An AMAZING thing to do is to comment on plosives( b,p,d,t heavy, drum, brash sounds), fricatives, ( f sounds), sibilants( s sounds) and liquids( l) sounds as this means you're analysing language at the same time as using technical vocab. So words like Flying, floating, fluttering, flitting, are a mixture of f and l sounds, they give the words an airy, elegant feeling.
OMAM main themes I can think of are probably kindness, friendship, American Dream etc. so that would be a good place to start. I know it seems endless, but they usually ask about a really key theme or ask you to pick a key scene. Let me know if you need any help.