A "hook" is a narrative device used at the beginning of an essay to grab a reader's attention.  In my classes, I call it an "attention getter."  It is a critical piece of any essay because it serves to motivate readers to keep reading.  If a reader is bored after the first sentence, then that reader isn't motivated to keep reading.  He/she may stop reading or he/she may let their mind wander instead of engaging with the writing.  

There are a variety of ways to grab reader attention, but I recommend 1 of the following 5.  

1. Ask a question.  This is an effective way to engage a reader's mind because it forces them to begin thinking of possible answers to the question.  
2. Make a bold, perhaps controversial statement.  This generally works well because it gets an emotional response from a reader.  I do want to caution that this might get reader attention, but it also might anger your reader against your essay's argument. 
3. Use a definition.  This seeks to activate the logical part of a reader's brain.  At the same time, it will cause a reader to ask himself/herself why a writer would provide that particular definition.  
4. Use a quote.  A quote tends to draw your reader in because people assume that a quote is important for one reason or another.  Your reader will want to know why that quote is important, so he/she will keep reading.  
5. Use an anecdote.  People love stories, and a story serves as an interesting way to get a reader reading and interested in continuing to read.  Be sure that the story relates to whatever argument your essay is going to make. 

For your particular essay on cyber bullying, you could define it for your reader or ask "How does cyber bullying affect people of all ages?"  You could start with a brief anecdote about a person who went through an extreme case of being bullied.  The anecdote could even incorporate a quote, and then the essay is combining two narrative hooks.