An excellent question!
An effective introduction needs to make the argument of the essay clear from the outset. Your reader needs to immediately understand how you will answer the question and what your conclusions will be.
First, when considering the question you need to identify the assumption in the question and its key terms.
For example, let us consider the following question:
To what extent can it be argued that the First World War was caused by nationalism?
Here the assumption in the question is that nationalism caused the First World War. You don’t necessarily need to challenge the assumption in the question, although top end answers may well do, but you need to identify and understand it.
The key term is nationalism, which you may want to define, especially if you are using an unusual definition of nationalism. For instance, you may want to argue that imperialism and militarism were intrinsically tied to national identity making the two hard to distinguish.
Second, we need to make it clear to the examiner what our scope will be. Do we want to consider the long-term or short-term causes of WWI? If so, this needs to be clearly stated. For the sake of argument, we’ll say that we’ll consider the long-term causes originating in the formation of the new Germany in 1871.
Third, we need to state our methodology – what other aspects the essay shall consider. For instance, the balance of power, AJP Taylor’s theory of German determinism, the role of “great men” in positions of power driving events, such as Kaiser Wilhelm. Remember an excellent answer shall have historiographical points peppered throughout it and always access its different points throughout.
Lastly we shall state the conclusion, in a sentence. This is so that the reader is guided throughout the essay and that the argument is clear and concise.
For example, here is an essay attributing all the blame at Germany’s door (not necessarily an excellent argument but hopefully the introduction is clear!)
The causes of the First World War are one of the most debated subjects in history and often historians have attributed its outbreak to the fervent nationalism that gripped Europe at the end of the nineteenth century. This essay shall define nationalism as “the infiltration of feelings of fierce patriotism into government policy, including militarism and imperialism.” In order to understand the wider picture, it shall examine events from the formation of Germany in 1871 until the outbreak of war in 1914. It shall conclude that whilst nationalism played an important role, other factors, such as the role of leading figures, the changing balance of power and the German sonderweg were of greater importance making the war pre-ordained upon Germany’s creation.