There are usually two or three things you have to write about in every text you write in the exam so make sure that you find and underline them when you analyse the task.
You also have to choose the right register (formal, neutral, informal) language for your email/letter and we decide which tone is the most appropriate by checking who is going to read your text. For example, if you write to a good friend, your language needs to be informal, but if you write a letter of application to the manager of a company, you want to choose formal English.