A good way of dealing with the dread that can take over when you are faced with the souble-dread of a blank page /screen and a word count is to start writing as you would talk. Imagine you’re in the pub explaining what your essay is about and write that down. You can worry about spelling, grammar, language, structure and referencing later, in the beginning it is important to get things down.
Also see: Write first, review later
(If you are nervous about the process of writing, speech recognition software might allow you to actually tell your stiry, doing the writing for you.)