Professional writing inverts most of what you learned in your undergraduate essays.  The audience shifts from a professor who has the time to look at your arguments over 20 pages to a busy professional who wants to know the conclusion first, not last.  Here are some basic principles of professional report writing.

The Major Goal of Professional Writing
To avoid reading and to streamline reading is the goal of professional writing:  this goal is very worthy when you consider how long it takes to read!  The techniques below show respect for a busy, decision-making audience. 

The Basic Structure of a Report

Ask at work for samples of well-written reports—take them home and study them.  You will find some variation, but most reports follow this basic structure:
Introduction
(sometimes an abstract, sometimes a transmittal memo or letter) that gives the reader the most succinct statement of the findings of the report.  Even if the abstract is 100 words, the Introduction will not go beyond 2 pages.  It has the most general level information.
Background
(sometimes called Statement of the Problem, Rationale, or some other heading that refers to the need for background information)
Body Segments
  You get to title your own headings for the body of the paper unless you have a set structure (that may include, for instance, Method, Analysis, Discussion, Results, Budget, Timeline, Conclusion).  I would urge you to be creative with headings:  always have a subject + so what (significance) instead of just a topic.  Help your reader avoid reading.
Appendix
  You can attach detailed information at the end of the report.  Anything you think the reader would like to have but not necessarily read while reading the report is fair game for the appendix.

The Structure of Information in a Report

In any professional report, information moves from

• most important to least important
• most general and abstract to very specific.

The most difficult thing to learn when switching to professional writing is putting the summary first at the top of each segment in 1 sentence.  Most people write how they usually write and then put the summary at the top.