You should begin a variable name with a lowercase letter because it helps distinguish between a class name and a variable name in your code. There are some other guides in the C# documentation as well, but we don't need to worry about them at this stage. Component names (class names) begin with an uppercase letter. For example, it's easy to know that Transform is a class and transform is a variable.
There are, of course, exceptions to this general rule, and every programmer has a preferred way of using lowercase, uppercase, and perhaps an underscore to begin a variable name. In the end, you will have to decide upon a naming convention that you like. If you read the Unity forums, you will notice that there are some heated debates on naming variables. In this book, I will show you my preferred way, but you can use whatever is more comfortable for you.