If you're just starting out, I'd recommend that you enroll in an online astronomy course to learn the basics. ASU26 and Coursera42 have excellent introductory astronomy courses. However, if you're already familiar with the basics and want to expand your knowledge, there are a number of sources that you could use and there is no "best source" among them. For example, you could use the OpenStax astronomy textbook25 along with online educational videos to learn astronomy, but for a more directed learning experience, you can always enroll in an online course.
Sai Charan
President, International Amateur Astronomical Association