As mentioned before, it is better to write the introduction after you have written the Results and the Discussion sections.
The Introduction should state the nature of the problem being investigated, and provide a brief review of the literature that is pertinent to the results and conclusions of the present study.
Do not provide any background that is not directly relevant to your results and discussion. Do not give every reference that relates to your work – just give the most important ones.
The last paragraph should describe the aims of the study, and you may include a brief (only one sentence) description of the major results. This means that the reader does not have to read all of the paper to learn the major findings.
An Introduction could therefore be structured into the following paragraphs:
(1) Background paragraph. This introduces the general topic, and shows why it is important.
(2) Paragraphs that give more specific relevant information. These focus the readers attention from the general topic to the more specific problem addressed by the study.
(3) Aims paragraph. This gives the scope and the aims of the study, and may also include a brief statement of the principal findings.
(4) A concluding paragraph. Often only one or two sentences, this gives the major findings of the study.