Have a read through these, they should give you an idea where to start.
I'll just say the 1st thing to do is brainstorm all your ideas and write them down. Then these links should show you how to go about it

http://www.english.bham.ac.uk/staff/tom/teaching/howto/essay.htm

http://www.aucegypt.edu/academic/writers/

http://www2.actden.com/Writ_den/tips/essay/index.htm

Good luck!