How can I make a thesis statement for my question, "Why are orangutans endangered?"

Your thesis statement is going to be an answer to your question.  I don't know if you have done your research yet, and this really is necessary in order for you to have a good thesis statement. Essentially, a thesis statement states your main idea and the points that you will use to support that main idea. In this case, your main idea is that orangutans are endangered, and your supporting points will explain why...

Your thesis statement is going to be an answer to your question.  I don't know if you have done your research yet, and this really is necessary in order for you to have a good thesis statement. Essentially, a thesis statement states your main idea and the points that you will use to support that main idea. In this case, your main idea is that orangutans are endangered, and your supporting points will explain why they are endangered.  This is not necessarily the form you need to use, but a thesis statement like this is a starting point:

Orangutans are endangered because of A, B, and C.

While I am by no means an expert on endangered species, I am going to take a guess that this is an endangered species because their habitat is disappearing, they are killed for meat, or they are taken by poachers who sell them on the black market, perhaps to serve some need in a particular culture.  You need to do the research to find out what the reasons are, and that will allow you to complete your thesis statement.

Your thesis statement should be the very last sentence in your introduction. It gives the reader an outline of your body paragraphs, then.  Each of your body paragraphs should cover just one reason the species is endangered, and these paragraphs should be in the same order as the reasons are listed in your thesis statement.  Give each body paragraph a good topic sentence, to let the reader know which reason the paragraph is about.  Support each paragraph with evidence, to show the reader about this particular reason for the problem. Then all you need is a conclusion, which should remind your reader what your main idea is and all the points, the reasons, that you covered in the body paragraphs.