This part of the React Tutorial runs from What Is React through to Your First Component. There are 3 lessons here, and working through them takes about 18 minutes at a steady pace.
This is where the React Tutorial starts. Nothing before it is assumed, and every example runs in the editor built into each lesson, so you can change the code and see the result without installing anything.
Each lesson below says what it covers before you open it. Read them in order the first time; afterwards this page works as a index you can jump back into when you need to check one thing.