Data Science is a multi-disciplinary domain that is a conjunction of many domains like Maths, Statistics, and Programming Languages with Machine Learning and Artificial Intelligence.
Now I am coming to your question, How do I learn data science online? I will suggest you enroll in a training course at a respectable institute like Intellipaat. The reason why I am suggesting that is because, if you are serious about learning Data Science then you must learn it practically. The free resources will only be able to give you a start or the basics which is a long way away from what real companies or markets expect from you. Check out the below video for better insight and then enroll in a Data Science Masters Training course from Intellipaat.