My advice is always to jump in and start building! My background is math originally, so I had some of the tools in my tool box, but I'm mostly self-taught in computer science and machine learning. I read textbooks, research papers, code repos, but most importantly I build a lot of stuff. Once I'm excited about an idea I'll figure out how to become an expert to make it a reality. Over the years the skills start to compound, so it also helps that I'm an old man!