This is a short and succinct YouTube video on critical thinking:
What is the most important lesson of critical thinking? What is it supposed to achieve? Bertrand Russell's interview here is quite relevant.
§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. Try searching for "critical thinking".
"Critical Reasoning for Beginners" from Department for Continuing Education at Oxford.