How do I use custom types?

I have created a type in its own cs file, and I want to use it to instantiate objects of said type in other files, but I don’t know how

public class MyClassA
{

}

public class MyClassB
{
    public MyClassA CreateA()
    {
        return new MyClassA();
    }

    public MyClassA CreateA_ByType()
    {
        return Activator.CreateInstance(typeof(MyClassA)) as MyClassA;
    }

}