If this is a serious, multi-page reports, such as testing, it is best to make the title page. For a small report just write the title on top, on the first page. After the word "report" in the title indicate the subject of the report period for which you are reporting.
In the introductory part describe the problem, goals and objectives of the work that was carried out by you. If this is the standard reporting at specified intervals - monthly, quarterly report on work, no prologue to write is not necessary - its essence already stated in the title.
In the main text of the report lists the work that you have been performed in the framework of the task and specify the timing of each item. Then give an opinion on how you manage to perform the tasks that were put in front of you.
Analyze why, if this was the case, you could not perform all that was planned. This could happen due to time constraints, lack of materials or lack of necessary equipment. List all the reasons that have influenced it. In fact, this part of the report is the most important, because here you have to list the objective reasons that prevented you in good faith to do the job. Thereby you shift the responsibility on management, which failed to provide you with everything you need.
Based on your report, the authorities obliged to draw conclusions and to take the necessary measures in order to provide you with all the necessary work materials and equipment, or to extend the performance of certain types of work.