I agree, following some good tuts on youtube, for example, is one of the most effective ways of learning PS as you can hear what the author has got to say as they often explain things very well.
I would like to add to this. Search websites for a certain effect you're wanting to learn. If its an article walkthru (you have to read it) simply take the search to youtube. You can learn loads there nowadays.
you have got so many good answers but better to do any professional course first then go to online available material.you will find many good videos on youtube .