This is a very big topic, and I think your biggest task is to narrow your focus. You will have no difficulty finding plenty of materials in your research. What aspect of swine flu might you want to discuss? The topic can be viewed from a public health perspective, for example, focusing on what public health officials have done to prevent the spread of the disease. Even that might be narrowed to focus on what was done in Mexico, or in the USA, for example. Perhaps you want to focus on how the media handled information about the flu. Did the media make things worse? Or did their reporting reassure people? Another possible focus might be what actions people should take to protect themselves. You could also discuss the biology of the disease, its combination of human, swine,and avian flu, and the possibilities for mutation. As you can see, there are many ways to approach this subject.
No matter what aspect of swine flu you choose to talk about, you will need to provide some background information on the flu, assuming that readers don't know much about it. This can be accomplished briefly in an introduction, or it can be accomplished in a separate paragraph, right after the introduction.
Decide what approach you want to take, and what points you want to make. Each point should be made in a separate paragraph. Those paragraphs are the "meat" in your "sandwich," with an introduction and conclusion holding your sandwich together.
When you offer your reader quotations, information, or ideas that you found through your research, you will need to provide in-text citations to let the reader know your sources, and a final page listing your references or works cited. Check with your teacher to see if you are supposed to use MLA or APA form. I have given you links to learn how to use both, if you are not familiar with these.