Any introduction should contain four parts: a research question, an identification of variables, a quantitative hypothesis, and a procedure.
The introduction should first contain a focused research question or the objective of the experiment. Do not rewrite the objectives provided in the lab description. Try to spark the interest of the reader, by trying to connect the experiment to real life examples or applications.
You must also identify the variables relevant for this experiment. Variables are factors that can be measured and/or controlled. You must explicitly identify all variables into three types:
Independent variables: are those that are manipulated, changed, during the experiment.
Dependent variables: are those influenced by the manipulation of independent variables and are measured during the experiment.
Controlled variables: such a variable is one that should be held constant so as not to obscure effects of the independent variables on the dependent variables.
You should not identify variables you will calculate from your measurements. In this section, you ONLY want to describe what you will measure. For example if you measure the position of an object (dependent) as a function of time (independent) while keeping the angle constant (controlled), then velocity and acceleration will be calculated from those measurements and you do not need to add them in the list of dependent variables.
The introduction should then present a quantitative hypothesis, based on the essential theory describing this experiment. Explain what you will calculate from your measurements and what the expected results are. If you are calculating a known quantity, state what the expected value is and the reference used. If you plan to make a graph, state what information will be extracted from it? Will the graph be a straight line? What should the values of the slope and intercept be, and what do they represent?
Finally the introduction must contain a description of the method (procedure) used to carry out the experiment. It is better to summarize the procedure in your own words with complete sentences, and not as a list of steps. It should be clear how this procedure makes an effort to control the different variables influencing your results. The procedure should be concise (giving a lot of information clearly and in a few words) but comprehensive. Do your best to avoid the imperative mood when writing this section, it will prevent your text from sounding like a list of commands.