Duke University's Free Coursera Course on Critical Thinking: Take it!

Duke / Coursera has a free course on critical thinking. It can even lead to a certificate if you take a couple other classes, but by itself it's a great intro to critical thinking and proper arguments, as well as detect improper arguments being used on you.

https://class.coursera.org/thinkagain-003/lecture

Class is free, though they're already on Week 8, so you'll have a bit of catching up to do!