It is useful to have experience explaining scientific experiments in a formal written report.  As you progress through science courses in high school and college, different teachers will have slightly different versions of how they want you to write their lab reports.  The guidelines I give you here provide a framework from which you can learn the basics of writing a solid lab report.  When I was in college and proudly handed in a lab report, a memorable old professor scrawled "Be bright, be brief, begone" in red ink across my title page.  I would give you the same advice when writing lab reports.
Title...Your title should be specific and tip the reader of what is to be expected.  For example, "My Chemistry Experiment" would be an inferior title compared to "The Oxidation of Zinc and Iron".
Introduction...This section explains the purpose of the experiment.  Why was the experiment performed in the first place?  In addition to stating the purpose or objective of the experiment, the introduction may provide background information and explain particular concepts or theories.  Often the introduction includes an hypothesis (or educated guess) that is to be tested in the experiment.  Avoid sentences that contain I or we.  For example,  I wanted to know if...or We thought that...would be less effective than The purpose of the experiment was to measure how much iron oxide was formed in the reaction...Passive (not active) subject sentences, past tense verbs.  Helpful link, go here...
Procedure...This section explains how you performed the experiments and what materials and equipment were used.  Do not use a step-by-step cookbook style.  What is wrong in a sentence with expressions such as "...then we added some iron filings to the test tube"?  It is helpful to include diagrams that explain how an experiment was set up.  If you used exact quantities of materials then be specific and mention exactly how much (metric units are almost always used).  Why might the statement "...6.5 grams of magnesium was added..." be better than stating "...Next I put some magnesium into one of the test tubes in the test tube rack.."?  Helpful link, go here...
Results...This section includes what specifically happened in your experiment.  This may include both qualitative and quantitative information.  Tables and graphs with proper legends are frequently used in this section but should not be included without clear explanatory sentences.  Be careful to only include results in this section.  It's tempting to interject procedural and discussion comments while mentioning results.  Why is the first result statement better than the second result statment?  "An average of 12.0 ml of gas was produced in the 5 experimental trials."   "After we put the magnesium in five times, we got twelve miligrams of bubbles".  Helpful link and examples., go here Discussion...This section explains the results.  "There was more more iron oxide formed in the third reaction because there was more oxygen available."  This section may allude to the introduction by answering the question(s) being proposed.  This section may discuss any glitches or problems with procedures.  This section offer concluding remarks.  Helpful link, go here...    or here...

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