A white paper should ideally be something between 3 and 14 pages long. If it is shorter, it is more like an article or a blog post, if it is longer, you’ll probably lose potential readers.
Your white paper starts with a captivating summary telling readers what they will find in the white papers, it lays out the problem you address in it and must also tell what help they will get from the paper. You need to make them interested in what you must tell. Readers who encounter the same problem will most probably go on reading in order to find the solution.
White papers should be based on a fair amount of data and should therefore contain quite a lot of text. It starts with the bigger picture where you expose the problem and then narrows down to the proposed solution.
If you mention your product/service early in the paper, they might very well be turned off, seeing it as an advertisement. If your white paper has a commercial purpose, you only mention your product or service at the end, because readers will then have read all the information, when they are already convinced by your solution.