Here are some helpful websites when working on your grammar skills:
Grammar Girl: http://www.quickanddirtytips.com/grammar-girl
OWL (Purdue Online Writing Lab): https://owl.english.purdue.edu/owl/section/1/5/
Grammar Revolution: http://www.english-grammar-revolution.com/diagramming-sentences.html
CCC Foundation – Guide to Grammar and Writing: http://grammar.ccc.commnet.edu/grammar/diagrams2/one_pager1.htm