It’s difficult to offer general advice because it depends on a number of factors:
A few tips I can suggest.
First, you should be getting IT training during the first day or two. This is a question worth asking in terms of the numbering format for your firm – ours is 1/1.1/1.2 etc.
Second, check with the person who sets you the task whether they have are particular style they prefer. If it’s a senior then I’d suggest asking your trainee buddy or a secretary for suggestions.
Third, try to understand what the task requires. It’s not usually so rigid – unless the person has asked for a particular format. I can’t say I’ve ever included purpose and rarely an intro/conclusion. Typically, I’ll do a summary of my findings (sometimes in bullet points) and then provide more detail in the body of the report. The key is that the partner/supervisor can get to the important points quickly. Then, if they want more guidance on how you got there, they’ll read the main body.
If you’re ever confused, fall back to the second point. It may even be good to make a start and check in with the supervisor/partner to see if what you’re doing is along the right lines. We do that a lot in practice. It saves you wasting time doing it incorrectly. If you can do that during a vacation scheme, it often shows a lot of professionalism.