How should I go about writing a thesis statement about stopping all forms of immigration in the United States?

In order to formulate a thesis about the topic of completely closing our borders to new immigrants, you will first need to choose which side of the argument you are on. Do you favor the total closing of our borders, or not? That is the first question you must answer, but not the most important.The second question you must answer, and the far more important part to any thesis, is the "why" or "because" statement. In this case, why do you believe that the United States should either continue to allow immigration, or why should we do the opposite? 

Think of your thesis statement like a math problem.

A thesis = your position + your reason why that position makes sense. 

For example, if you were to advocate for keeping our borders open and allowing legal immigration, you might compose something like this:

"As a country founded by immigrants and enriched by diverse cultures and points of view, the United States must continue to welcome the brightest minds from around the world, in order to maintain a competitive edge in this global economy."

Remember that whichever side you take, your thesis must succinctly express the main thrust of the argument that you are going to flesh out in the rest of your essay. There are many reasons why you might pick either side of any argument, but a good thesis (and a good essay) results from the writer having a very specific argument as to why his or her position is correct.

So, only pick a thesis that you can cogently support with specific evidence and argumentation. A clearly stated thesis will act as a guide for your entire essay.