New to the Planes or this website? I recommend starting right here with this series of four articles that introduce the key concepts of Planescape and how to add the planes into your game. After that, check out my other posts or the published materials and other resources available online.
Introduction to Planescape 1/4: What is Planescape?
Introduction to Planescape 2/4: D&D's Unified Theory of the Planes
Introduction to Planescape 3/4: Understanding the Core Setting
Introduction to Planescape 4/4: Adding Planescape to Your Campaign