Robotics is a massive field and comprises of not one but several concepts.
You first need to outline your own prerequisites. For instance, what is it that you are most comfortable with?
Whether you wish to be a programmer or a motion planning expert.
How you choose a course would also depend on your current skillset and qualification.
There are various courses available online for you to pursue and get a clear idea concerning where you fit in with respect to your skills and interest.
Jack of all trades, master of none. This speech is totally true for robotics engineer. You have to know a bit of everything (related to robotics) rather than being expert in one area. Actually, robotics doesn't look like other traditional jobs. For example, if you want to be a lawyer then study law, if you want to be an interpreter then study languages, if you want to be a doctor then study medicine, if you want to be a robotics engineer then well.... okay, let's give an answer with details.
It is very necessary having a good understanding of STEM which is Science, Technology, Engineering, and Math. If you're in high school try to spend more time on physics and math because physics covers lots of useful topics that used in robotics such as mechanics and electronics. Math is one of the very first subjects that help you a lot to apply your knowledge to problems. For example, good knowledge of math helps you to analyze electrical circuits and to calculate kinematics of robot body.
I guess everyone will agree robotics is a branch of engineering disciplines - mechanical, electrical, and computer engineering (or computer science).If you are looking for online courses you can try this