As for online courses specifically I'd say Victor Cheng's workshops are pretty clear and helpful (https://www.youtube.com/playlist?list=PL8b_fmdDHHyCznYmSeWJrdrJN4UJhUrsh). There's also a number of audio recordings of case examples with his feedback and explanations called Look Over My Shoulder, quite popular among candidates.
However, the most helpful for me at first were the books: Case in Point (ideal in the very beginning, very simple and clear with lots of short cases), Crack the Case and Ace the Case.
And, most importantly, it's all about practice in the end. Once you get the usual case structures, it's best to get face-to-face case experience (or via video these days), that'll be the closest to the real interview experience.
Hope this helps, feel free to reach out if you have further questions!