If you are writing blog posts and actually copying content from other sites – you’ll not be doing any good (if not bad) to your blog from SEO perspective. The math is simple – if the content is not yours – do not put it in your website.
Now the question is how are you going to get unique contents? Simple – write your content yourself. To make sure your content is not matching that with someone else’s – you may check your content with Plagiarism checker. While there are standalone system for plagiarism check – there is a plugin for WordPress – which does it right from the wordpress editor. Moreover, it is FREE.
However, if you are not working with WordPress and would like to use something like this – here is a link where you can do so: http://smallseotools.com/plagiarism-checker/