There are two aspects, the tools and the knowledge. What people recommend above will just teach UI (How to build), but not UX (why to build it this way and how

There are two aspects, the tools and the knowledge. What people recommend above will just teach UI (How to build), but not UX (why to build it this way and how to approach the problem). For the tools, it's like in software engineering, you need pick the your "stack" (which wil depends on the goals and deliverable). Do NOT use Paint / Photoshop to build mockups, you will waste your time and hate your job. Get a Sketch License, find a good tutorial online (Udemy and co). To master the field, study the fundamentals: cognitive sciences, human factors, cognitive ergonomics and design patterns, the goal is to understand how the brain works, perception, memory, decision making, limits... Basic computer science is going to give an edge. Basic knowledge in experiment design will be necessary to test your design with rigor (UX Researcher).