1. Google, https://www.google.com/. (Links to an external site.)It organizes the world’s information and make it universally accessible and useful. Google provides me a lot of useful learning information. During in school, sometimes we didn’t get what the professor say, or we may want to explore one subject deeper. I use Google a lot helps me improve the ability to do my self learning.
2. Kaggle, https://www.kaggle.com/ (Links to an external site.). Kaggle is is an online community of data scientists and machine learning practitioners. For our data analytics major students, we use it a lot. It either give us the newest dataset or the perspective of improve one model or one graph.
3. LinkedIn Learning, https://www.linkedin.com/learning/me?u=2185266 (Links to an external site.). LinkedIn Learning is an online educational platform that helps you discover and develop business, technology-related, and creative skills through expert-led course videos. Personally, I use it as my self-training tool. It taught me not only technical skills, but also the soft skills, such as communication skill, team work skill. 
Those three tools are my favorite, and I almost use it every week. I want to share it with you. Hope they will give you some insight about online study tool.