How do I start writing an essay?

With any English essay you must start by reading the text at least twice and then go through it a third or fourth time, highlighting important passages, phrases or words and making notes. What you write here will be the backbone of your essay: the key features you've spotted that show you understand the text. 
Once you've located the important features make sure you go through exactly why they're important: what effect do they have on the reader? How does it affect our understanding of the characters? Does this add humour to the section? Why has the writer used this technique/included this feature in their writing? 
Once you've finished annotating and expanding, select 3-6 (depending on the size of your essay) of the main points to use as the main arguments for each paragraph and then assign the other features to each section to use as supporting statements for your main argument. 
Once you've finished this, you have the bulk of your essay written: all you need to do is to find the extracts from the text to support each point and expand on everything to create a water-tight argument. Write your essay using the PEEL structure: Point, Evidence, Explanation and Link.