The very best writing and editing on –line help source is the Purdue University Online Writing Lab OWL):

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

See, especially, their material on:

The Writing process: http://owl.english.purdue.edu/owl/resource/545/01/

Grammar and Mechanics: http://owl.english.purdue.edu/owl/resource/539/01/

See also detailed writing edit samples from commercial editors at:

http://www.editorsforstudents.com/samples.html

http://www.academicedit.com/editingsamples.htm

http://www.authorspublishing.com/EditingSamplesIndex.htm