When you write code, it is important to get into the good habits of organizing your code. While there are no specific rules about how and where to place your code, there are definitely conventions. (Conventions are suggestions that most programmers follow. This way other programmers can quickly and easily understand their code.)
The most common organization is to have one main folder with your html files in it. There are also subfolders (folders inside the main folder) for your CSS files and image files. (In this example I also have JavaScript files.)