"How do I start an essay?"

Before starting an essay, writing a brief plan is always useful - even if it is just a bullet pointed list. It is important to make sure your plan covers all of the relevant assessment objectives: AO1 (understanding, reading, and responding to the text), AO2 (language and structural analysis), AO3 (showing how the text relates to the historical context). For example, part of a plan for the essay "How does Shakespeare use religious imagery to shape meaning in Romeo and Juliet?" could be as follows:
Introduction: express the importance of Christianity in Elizabethan society (AO3)1st paragraph: analyse some language with religious imagery in the given extract and relate it to similar imagery elsewhere in the play, e.g. "swear by thy gracious self, Which is the god of my idolatry", "O, speak again, bright angel!" (AO2)2nd paragraph: suggest why Shakespeare might have chosen to use religious imagery and explain what it means within the context of the plot (AO1)3rd paragraph: suggest how contemporary and modern audiences would react differently to this use of imagery (AO3) and explain how this is important for the plot (AO1) e.g. a contemporary audience may consider it blasphemous when Juliet compares Romeo to a god, potentially exposing that her obsession with Romeo went too far and led to her downfall.
If you are still struggling to get words onto paper, practice using some easy sentence starters as a boost towards the argument you want to make, for example, "Throughout the play...", "[the writer] employs a motif of...", "In the [time period] it was commonplace for..." etc. Alternatively, a fail-safe way to write an opening sentence is to simply rewrite the question, so, the example question above would become "Shakespeare uses religious imagery throughout Romeo and Juliet to shape meaning."