Generally speaking, your opinion should be stated in the introduction, which is the first paragraph.  In fact, your position on an issue will be your thesis statement.  When you have decided what points you want to make to support your opinion, you should also make a statement in the introduction that gives your reader a "preview" of the points you will be making.  For example, if you were arguing against the death penalty, your statement might be in this form: "The death penalty should be abolished because of A,B,and C." 

The body of the essay should consist of paragraphs that support the points you wish to make, each paragraph having a topic sentence that is the point of the paragraph and all the sentences in that paragraph supporting that point. 

The last paragraph is the conclusion.  It should restate your argument and review the points you have made.

If you think about it for a moment, you will see that concealing your argument until the end makes it difficult for the reader to follow your ideas and assess them. Can you see the reader asking, "Where is she going with this?"  That is why you need to state your opinion in the beginnning. 

When you are writing a narrative, for example, an autobiographical or descriptive piece, you do not want a thesis statement because you want the narrative to unfold gradually for the reader.

I hope this helps.  Good luck to you.