Textbook is good to understand the grasp definitions and concepts in relation to your area of study. HOWEVER! Don't rely on it to help you in an essay!!! Especially if you have an older textbook, some of the material in there can be outdated and slightly undercooked in relation to the concepts. Case studies are the best thing to have; especially articles by Ross Gittin's in the SMh on Saturdays. He writes things in relation to what you study now, so if you can find them, READ THEM! They are incredibly easy to understand, and they explain economic terms and concepts in explicit detail. Articles by The Economist, The Australian etc are great as well; if your library gets newspapers every day, I suggest you spend 10 minutes of your free period or lunch reading some of the material, as it can help.
For me and our class, we always had class discussions on different concepts and ideas, and current economic issues. That way we could express our opinions but also get other ideas from other kids in the class. If your class doesn't do class discussions, maybe find a buddy who you can discuss these things with. It seriously does help!
Also, learn how to write economics essays! One of the things I struggled in Year 11 with economics is that I would write an english essay in economics. Economic's essays need to be precise and descriptive; Don't waffle because you will most likely lose points for it. At our school, our principle (also my economics teacher) introduced a Personal Best or PB20 Challenge where you had to attempt to do 20 essays (at least 4 from each subject, and at least 1 from each area of study) and I managed to do 6 extra essays on top of the 4 required and I got a 19 out of 20 in my trial exam. It was on a ferocious catholic paper too so I was pretty stoked...
Mostly, practice is the best way to learn economics. I loved economics, and am now going to Macquarie Uni to do Business Admin with Economics... If you do the extra bit here and there and go into more depth about certain concepts and ideas, it will seriously benefit you in the end. Economics is highly ranked by the Board of Studies, and anything over 75% will seriously improve your UAI.
Good luck with your HSC, and have fun in Economics...