This is a short and succinct YouTube video on critical thinking:
§1. Online courses
There are now many online courses on critical thinking. They should not be difficult to find. Here are two you can try out:
Kahn Academy has a course on critical thinking with videos and exercises.
https://www.khanacademy.org/partner-content/wi-phi/critical-thinking
"Critical Reasoning for Beginners" from Department for Continuing Education at Oxford.
http://podcasts.ox.ac.uk/series/critical-reasoning-beginners