Plagiarism is very easily avoided: don’t present the work of others as your own. To elaborate, if you discuss ideas or reproduce words that you’ve seen in the work of others, you must clearly attribute those ideas or words to their original author.
A simple citation and a few quotation marks can save you a whole of academic pain. Witness the furore over the allegations of plagiarism levelled against Germany’s former Education Minister (The Independent, 9 February 2013), who strongly denies the accusations but resigned anyway.