A field is just a class variable.

Defining a field

You do it just as you would do a variable, except it within the scope of the class.

class ExampleClass
{
	public int x;
}

Using a field

You do is just as you would do a variable, except you have to go through an instance of your class to do so.

class Program
{
	public void main()
	{
		ExampleClass ex = new ExampleClass();
		ex.x = 5;
	}
}