The introduction can be relatively short. Start out with some general (but not too general) statements about the topic. Lead up to the last sentence, the thesis statement, which should include exactly what you are going to talk about. By that I mean, are you going to talk about three causes, two causes and one effect, two effects and one cause... you have flexibility here. Then each body paragraph should start with a topic, the cause or the effect you are going to discuss. 5 sentences minimum, topic, 3 sentences to explain details and examples and a concluding sentence. Again Conclusion can be short. No new information here, just summarize without repeating yourself, the basics of what you have written about.

i have read , that there are three things in the introduction which are the topic sentence, background sentence and thesis sentence . am i right ? .

The introduction to your essay should include a thesis statement. It should include what you are going to write about, basically. The body paragraph should include the meat of your essay, or facts that support your thesis statement/introduction. The conclusion is merely a summary and ending to your essay.