To get you started with template creation, see:
http://word.mvps.org/FAQs/Customizat...platePart1.htm
A watermark is a background image that can appear on all pages. If you only want it to appear on the first page, configure your document with a 'different first page' layout before adding the watermark (for which, see Page Layout|Page Background>Watermark). You can achieve much the same result by accessing the header and using Insert|Picture. A different last page will require a 'Next Page' Section break before it. If you create that after inserting the watermark, the last page will inherit it; otherwise, if you unlink the Section header from the previous Section, the last page can have its own watermark.
As for turning a document into a template, you do that via 'Save As' and making the 'type' a template (probably in the dotx format, though you should use a dot format if your colleages using it don't have Office 2007 or later).