In a debate, an opening statement must set out the basics of your argument. It must let the audience know what side you are taking in the debate and it must give a number of arguments for your side. In order to write an opening statement, you must first know what you are going to say.
Although you do not state this explicitly, I take it that you have been assigned to support the statement given in your question. If so, you definitely have the harder position. You will need to argue that it is good for our country to move up through the levels of the demographic transition. Let us look at three arguments that can be used to make this case, noting that I do not necessarily believe any of them.
First, you can argue that banning immigration will lead to more jobs for people who are already in this country. You can argue that immigration increases the supply of labor and, therefore, drives down the wages that workers can demand. It also creates more unemployment than would otherwise be the case.
Second, you can argue that a declining population will make the average American better-off. You can argue that the money that we have will not need to be split up among as many people. If we have fewer people, the money that, for example, the government has to spend on various programs can go to fewer people. That will mean that each person who gets money will get more of it.
Finally, you can argue that a declining population will be good for the environment. You can say that we will not keep needing more housing. This will reduce the need for urban sprawl. If this happens, there will be fewer cars driving and emitting greenhouse gasses. Fewer people will mean less need for electricity. It will mean less need for water. All of these things will help to make our environment more sustainable.
So, to write your opening statement, write these arguments, and/or others that make similar points. You should also have some sort of introduction. You might say something like “Our world today is in grave danger due to environmental problems. At the same time, our government is running out of money. We can help solve both of these problems by banning all immigration to the United States.