Sitting down at your desk in the exam hall can be extremely daunting. Months of preparation, cramming and anxiety have lead to this moment and it's natural to feel overwhelmed and panicked. The most important thing is to stay calm.
Take a few deep breaths, clear your head and when you open the exam paper, read the questions carefully. Do not just rush towards the first one that looks appealing - sometimes a question that looks really difficult at first can be cleverly picked apart and analysed when given a bit of thought.
Ignore what everyone else is doing around you. If the girl next to you looks like she's writing 100 words a minute, don't panic. The most essential thing to remember when you sit down to do your exam is to PLAN. It can be really tempting to just spew everything you've revised onto the page as quickly as possible, but this is the worst thing to do. You'll end up with an essay that is disorganised, frantic and veering off course.
Jot down the first things that come to mind, the basic points and quotations that will be useful for the question. When you've quickly brainstormed this, make a structured plan, detailing (in a sentence or two) what you are going to write in each paragraph. This will limit any panicking, prevent waffling and ensure that you have a coherent, flowing essay.
This being said, timing is everything. If your exam is an hour long, do not spend any more than 8-10 minutes reading the question and making the plan - you want to optimise the amount of time you have to write a sophisticated essay.
Ultimately, to stay calm in an exam situation you must completely ignore what everyone else is doing around you. It's easy to panic when you see that smart guy in your class asking for another sheet of paper when you're only half way through your first. Remember that everyone works at a different pace and if he hasn't taken the time to gather his thoughts and structure his essay, his work will be lengthy but jumbled and rushed. Don't be scared to ask for a spare piece of paper for your plan at the beginning of the exam - having this beside you is a lot easier than writing it in the back of the answer book, for example.
Take deep breaths, gather your thoughts and read the question carefully. Try not to panic and don't be tempted to start writing the essay immediately. Plan, plan, plan - it might seem tedious, but it will prevent your brain from veering off course and producing a messy, rambling and unfocused essay.