Don't expect that you'll read a prompt and the ideas will flow - they didn't for me! You've got to brainstorm by asking as many questions as you can about the prompt (
this post might help you). You don't have to start off with ultra-awesome ideas - you've still got a few months to practise getting better and better! A really great way to study is getting a
bunch of prompts and brainstorming/questioning each one thoroughly, trying to come up with ideas and evidence from your text/the world.
If you're looking for external evidence to back up your ideas, check out the
~*Context External Examples Guide*~. You really have to do some research, like pick say 10 examples from anywhere that you're interested in (it honestly doesn't matter what they are, as long as they're not Hitler or something); research them, write notes on them, and suggestions of how you could apply each example to different types of prompts. Then try to use some of those in every essay.
(This isn't great advice just before a SAC; when your SAC is over, try to research external examples and brainstorm heaps of prompts, for your next context SAC and the exam).