Think of the lab report as a scientific essay.  It will contain the following sections:

TITLE:  The title should summarize, as specifically as possible, the subject of the lab.

PURPOSE:  A single, concise statement of the major objective(s) of the lab that answers one of the following:  What question or questions is this lab exercise designed to answer? OR What is this lab exercise designed to show or prove? OR What hypotheses is this lab exercise trying to prove or disprove? Note:  To make a hypothesis, you must be able to provide support for that hypothesis, based upon your knowledge or literature research.  A hypothesis should be an EDUCATED prediction!)

PROCEDURE:   This paragraph should not be included unless you have been asked to explain your procedure.
If asked to do so, you should include the information necessary to allow someone to repeat what you did, such as:
-What data was used?  Include information about the data such as the data source (i.e. Internet, observations, models)
-Include definitions of key terms and anything else necessary to understand exactly what was done.

DATA: This is a record of all measurements and other observations encountered during the lab exercise.
Record data carefully and immediately.  Do not rely on your memory!  The most carefully made observations are not useful if you do not record them accurately.  Always include the units of physical quantities.  You and your lab partner(s) should complete all the work and make all of the observations together.  It is very difficult to write a good lab report if you have not done some part of the experiment!  Do not copy the results of other groups unless instructed to do so:  To do so will be considered plagiarism!  If you and your lab partner(s) disagree about an observation, record your own.  Present the data in a way that makes it easy to discuss and to infer conclusions (Tables or graphs are often helpful!)  Do not describe the procedure or discuss the data in this section -- just present your data!  Graphs will sometimes be required or helpful.  These should be drawn on graph paper or with a computer.  The horizontal axis should contain the independent variable (the one that is known), and the vertical axis should contain the dependent variable (the one that you are observing or measuring).  Each axis must be labeled.  The graph must be titled.  The intervals on each axis must be consistent.

RESULTS:  Separate from your lab notes, this section should contain a summary of the final data, presented in a form that is most useful for interpreting the results.  A short paragraph should be sufficient, along with any relevant charts and graphs labeled will. Remember to title and provide legends for all graphs and tables.  The graphs and tables should be comprehendable independently of their association with the text.  Results are usually dominated by calculations, tables and figures; however, you still need to state all significant results explicitly in verbal form, (i.e. Using the formula for slope, the rate of change of the data is V = 1.8m/s).  Graphics need to be clear, easily read, and well labeled.  Number and Title tables and graphs.  An important strategy for making your results effective is to draw the reader's attention to them with a sentence or two, so the reader has a focus when reading the graph.  In most cases, providing a sabmple calculation of each type is sufficient in the lab report.  Be sure to point out trends and identify any special features.  State your key result in sentence form.

DISCUSSION:  This is the most important part of the lab, as it is where you interpret your observations and results.  For each conclusion you should discuss its significance and whether or not it seems reasonable.   In addition, be sure that you answer any questions that were asked in the laboratory instructions. You should infer conclusions from your data.  For each conclusion, you should provide evidence from your data.  Be sure to include a sample calculation of each type used.  Give explanations for and implications of any relationships observed.  (When interpreting a graph, infer conclusions based upon the slope of the line or lines.
Support your ideas with specific, quantitative references to the results of your analyses.  How do your observations lead to the conclusions you reached?  Were the relationships what you expected from the underlying physical principles?  If a conclusion does not seem reasonable, can you find any error in your procedure which could have affected your results?  For each conclusion you should discuss its significance.  Address any interesting questions you may have had as you were working through the lab exercises.  In addition, be sure that you answer any questions that were asked in the laboratory instructions.  Can you make any generalizations?  Why or why not?

CONCLUSION:  Summarize your results, the main points of your discussion, and how they relate to your stated purpose of the lab.
You are to restate all the conclusions that were made in the discussion section.  No discussion or explanation or speculation should occur in this section! (Do not use the words "because" or "since" or "therefore".)  It is a good idea to include how the main points of your discussion are connected in order to demonstrate the overall significance of your findings and the concepts you learned.
Please do not tell me how much you enjoyed this experiment or how much you learned from it.  Just give me your conclusions!