How to create a subclass in C#?

Do you mean this?

public class Foo
{}

public class Bar : Foo
{}

In this case, Bar is the sub class.

Here is an example of writing a ParentClass and then creating a ChildClass as a sub class.

using System;

public class ParentClass
{
    public ParentClass()
    {
        Console.WriteLine("Parent Constructor.");
    }

    public void print()
    {
        Console.WriteLine("I'm a Parent Class.");
    }
}

public class ChildClass : ParentClass
{
    public ChildClass()
    {
        Console.WriteLine("Child Constructor.");
    }

    public static void Main()
    {
        ChildClass child = new ChildClass();

        child.print();
    }
}

Output:

Parent Constructor.
Child Constructor.
I'm a Parent Class.

Rather than rewriting yet another example of .Net inheritance I have copied a decent example from the C Sharp Station website.