Go through the ROS tutorials, using a simulated robot at first, then buy a tiny AMR kit and do the same thing IRL. Once you have those basics, you can ask yourself into which direction to go next, the obvious choices being 1) deeper into the research of the new-fashioned (bi-)manual manipulation (arms), or 2) more into business and actually build a real-world application for your mobile robot (which will involve a lot of tinkering with hardware). And +1 on what brudgers said. It's a hobby, so have fun.

If someone wants to get into robotics as a hobby for the first time, and the #1 thing you tell them is "start with learning ROS", one questions whether you are trying to help them or sabotage them.