I have created a workflow that contains a collection of ways how to learn KNIME that I came across over time. I tried to classify them by method of learning (video, self-paced) since there are so many ways in which you could approach the task of learning KNIME.
Besides that the best way might be to find a task you really care about and bring that to KNIME with the help of the mentioned resources and also the always helpful KNIME community.