The answer to this question greatly depends upon what your essay is going to focus upon. When writing about William Shakespeare's play Romeo and Juliet, there are many different ways one can focus an essay.
If you are writing about one of the themes of the play (love, violence, impact of society, and fate) you will need to focus on only one. If you fail to do this, the paper will lack narrowness and become to large of a project/idea.
If you are writing about one of the themes (let us say love as an example), a good way to begin the paper is to engage the reader by pulling from their own prior knowledge.
Therefore, one could begin an essay on love as seen in Romeo and Juliet in the following way:
The iconic balcony scene, from one of the most revered love plays Romeo and Juliet, is one which evokes feelings many people think about when they think about true love.
Another way to begin an essay on Romeo and Juliet is to find something that no one has talked about regarding the play. TO do this, think about something which you (personally) found intriguing or confusing. By examining this intrigue or confusion, an essay on this idea may turn out to be a novel one.
In order to begin an essay in which you are offering your own personal interpretation on the play, you will want to bring the reader in by alluding to the fact that you have a new, unexplored idea about the play. An opening to an essay like this may resemble the following:
Romeo and Juliet is not a play about tragic love or the impact one's family has upon the decisions one makes in life. Instead, the play is meant to bring out...
This thesis engages a reader's interest by telling them that the play is not what it has been historically recognized as.