You should write both the introduction and conclusion of your paper after you write the body of your paper, because the body of your paper is the main argument of the point you want to make. You need to know what your main argument is before you state the purpose of your argument and draw any conclusions.
That isn't to say that you shouldn't have some idea of what those two paragraphs are going to say. Before you begin, jot down some general notes about the argument you're going to make, what you want to talk about to prove your point, and what you hope to conclude from your argument. Keeping these notes general will make it easier to adjust your thinking as you do your research and put together the main body of your paper.
After you have the body of your paper written, you can then craft your introduction and conclusion — based on the notes you jotted down earlier and the body of your paper. These two paragraphs really go hand in hand, and should be entirely supported by the body of the paper. The introduction should clearly state your argument (for example, that your paper will prove something), and then should outline the points of your argument (for example, the three things that you're going to show to prove your argument). Your conclusion should then build on the introduction by showing how you successfully made your argument.
One word of warning about conclusions, though: Don't simply regurgitate your introduction; try to show what you learned from doing the paper.