Minecraft wiki entry for Creating_Forge_mods, associated text: Class files are specific to the Java programming language. A few Java tutorials to try are w3schools (web and mobile), SoloLearn (web and mobile), and kodevelopment(web). You will need an IDE (Integrated Development Environment) such as IntelliJ or Eclipse to read or create class files. This tutorial will focus on IntelliJ IDEA Community Edition 2019.
JSON files are a means of detailing the characteristics of objects used by Java class files. JSON is much simpler than Java. You will need a text editor such as Notepad++, Vim, or Emacs to read or create JSON files. You probably already have a basic text editor on your computer, but there are many advantages to using other ones instead.
Image files you may be familiar with include .bmp and .jpg formats, but Minecraft requires .png format (example: yournewlogo.png) in multiples of 16 pixels square (example: 16x16, 32x32, 64x64). You will need an image editor such as Paint.NET or GIMP to edit or create .png files. You probably already have MS Paint on your computer, but GIMP has so much more functionality, and Paint.NET is quite user-friendly. There are also several websites with tools for creating pixel art.