There are many reasons why teaching ESL students how to argue is important. First, it's important for them to think critically in the workforce and make decisions based on facts and reasons instead of simply using their own emotions. Secondly, being able to articulate clearly a fact and learning how to support that fact with data is an important part of being able to function in a work environment.
Here's a link to a Dartmouth University's writing lab with lots of information about argumentation. http://writing-speech.dartmouth.edu/teaching/first-year-writing-pedagogies-methods-design/teaching-argument
ReadWriteThink.org has a great resource for teachers to help students distinguish between persuasive writing and fact-based argumentation. http://www.readwritethink.org/professional-development/strategy-guides/developing-evidence-based-arguments-31034.html
Another good resource that addresses the specific needs of ELL students is here. http://www.ascd.org/publications/educational-leadership/apr14/vol71/num07/Teaching-Argument-Writing-to-ELLs.aspx
If you are looking for a resource for topics to argue, here's a great resource that has current events that are easily argued. http://learning.blogs.nytimes.com/category/student-opinion/
Here's another site that has the pros and cons of 52 controversial issues. http://www.procon.org
This site gives a long list of teaching resources concerning argumentation. http://www.webenglishteacher.com/argument.html
If you are interested, there are a lot of MOOCS that address this issue.
Have fun and good luck!!