The introduction to your report should provide background knowledge about the subject and a clear statement of the objectives of the project. This means that you have to do some research on the subject (yes, open a textbook or even take a trip to the library), recall what you actually did, and then get it down on paper in a clear and logical fashion. It is sometimes good to include two subsections in your Introduction: Background and Objectives.
The Introduction helps the reader understand the subject. The Background subsection should convince the readers that the subject is important, and should provide them with enough information to understand the results and analysis that follow. The Objectives subsection should state clearly what you hope to accomplish in the project.
Remember, scientists generally do experiments to test theories, seek evidence to support hypotheses, or determine measurable effects of various inputs on physical systems. They never do experiments to prove or to show an expected result. For example, you may say that the objective is to determine how the hardness of an aluminum alloy varies with the amount of cold work. Do not say that the objective is to prove that hardness increases with the amount of cold work. Similarly, engineers generally run tests to determine the performance of some system under controlled conditions, or the response of a system to variables, or the properties of the system (e.g., materials), etc. The engineer's purpose is often to learn how the system behaves so that this information can be used to guide analyses, modifications or future designs.
If you are designing or redesigning something, the need or reasons for the design should be stated in the Introduction. Do not state the properties of the object but rather the motivations behind the creation of this new object. The Introduction can include an overview of the reasons for failure of the old design, the new use of the object that is causing problems or the research that has proven the need for the environmentally friendly self-propelled follicle enhancer (bald spot paint in a can).
Once you have identified your objectives, you should make sure that the following sections all serve to show that the objectives were met, or, if not, why not. For example, if one of your objectives was to determine how the hardness of a material varied with the amount of cold work, the following material would be expected: a graph of hardness versus amount of cold work in the Results, a description of the metallurgical mechanisms involved with cold work including their effect on hardness in the Discussion, and a statement about the form of the variation in the Conclusions. Or, if the 0.1 litre per hundred kilometre family car was the (unattainable) objective, the performance prediction section of your report should clearly show why this is impracticable.