The length of the longer answer essay questions you have to write will vary between exam boards but some key things to consider are relevant to all of them which are useful for most essay answers in geography. Firstly, a very quick two-minute plan before you dive in can be helpful. Just jot down key words you need to include or perhaps just note down a list of key points which you can base your paragraphs on. This helps to prevent you from having to go back to an idea that you addressed in your first paragraph later on because you forgot something that you wanted to include, making a clear and orderly structure easier. At the start of the essay, I tend to give a very brief one-line introduction that directly addresses the question and makes it clear to the examiner that I am enganging directly with what they want, and you can also quickly list the key points that will form your argument, i.e. the themes of your paragraphs.
In the body of the answer, try and clearly divide up your ideas if you can. One of the most common ways of doing this would be to have a paragraph each on social, demographic, economic and environmental impacts/factors or another similar geographical catagorisation. This is useful as it gives a coherent structure to your answer and also makes it easier to perhaps give a judgement in your conclusion (depending on the question) and point out that one type of impact/factor was more important etc. Finally, the answer should end in a quick, tidy conclusion. Don't introduce any new ideas, just briefly sum up the content of each paragraph before giving an overall statement or judgement.