Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [supervisor or human resources rep],
I would like to report the theft of my [list item(s) stolen]. I believe the theft occurred [give approximate date and time range]. I always keep the item [standard location], and the last time I saw the item was [date and time]. The item was kept [locked/unlocked], and the following people would have had easy access to the item: [list individuals with access if locked, and areas with access if unlocked].
I would appreciate if you would investigate further into this incident. It is disappointing to consider that one of my coworkers would be dishonest enough to take something that doesn't belong to them.