How different is alevel geography from gcse geography?Watch this thread
In terms of topics you'll be studying three core units: Water and Carbon Cycles, Changing Places, and Global Systems and Global Governance. You'll also have three optional units: one unit from landscape geography (Coasts, Hot Deserts or Glacial Systems), one unit from either Hazards or Ecosystems under stress, and one of either Urban Environments, Population and the Environment or Resource Security.
I don't think there's any need for you to be worrying about essay structure at this stage, your teachers will guide you on this, but the general advice is an introduction, 3/4/5 paragraphs (this depends on your writing style), and an evaluative conclusion. Don't be worried if your first few essays receive low marks, it's a skill that will develop throughout the two years.