You just started Year 11. Silly mistakes are perhaps, the most 'normal' thing for someone who's basically just started Ext 1.
And Year 11 is also the year where you start minimising these problems. Inevitably, you're going to make some silly mistakes (unless you put in so much effort to becoming state rank material). You shouldn't expect to be getting 100% that often anymore.
So what do you do? You start focusing on how you make the silly mistakes. Somewhere in the forum Jamon talks about all sorts of silly mistakes, and you should consider writing down those that are relevant to you. Then, when it comes time for preparation, keep that checklist in front of you as you do past papers, so that you know to watch out for those things. By looking at the checklist for a sufficient amount of time, you'll start memorising them off by heart a bit more. So in the exam, when you're done and it's time to check your work, run through the checklist in your head, making sure that you haven't done the same mistake again.