How do i begin to write an essay

First you need to carefully read the question. Pick out the important words which describe how they want you to answer the question. For example "How did the assassination of Arch Duke Franz Ferdinand contribute to the beginning of WWI". Here you main topic is Arch Duke Frans Ferdinand. But they want to know his contribution which suggests that they also want you to compare it to other contributing factors such as the already prevalent political turmoil. Because they have mentioned the Duke by name it would be a good idea to keep referring back to him as a means to justify other contributing factors.
Next you need to make a plan. This should take you no more that 5 minutes. The main purpose of this is to keep your writing on track and to stop you from waffling about too many topics. An easy structure is to have an argument for your introduction then three main paragraphs each of a different point and then conclude summarising what you have already said.
Once you have this remember to keep referring to the question, using the wording to keep you on track.