Many different print and web resources provide tips on how to write short essays and models of good essays that you can imitate. A good staring point is the Purdue Online Writing Lab or OWL:

http://owl.english.purdue.edu/

Another way to look for technical advice on how to write essays is to think about the specific skills you are attempting to develop. For example, if you are interested in style, or figures of speech, Silva Rhetoricae, has comprehensive information on style as well as very solid sections on arguments and organization:

http://rhetoric.byu.edu/

Many university web sites have material on writing academic essays. Another useful resource, covering research, conventions of academic writing, and structure of essays can be found at the University of Toronto website:

http://www.writing.utoronto.ca/advice/general/general-advice