Could some one help me? What formula do I need to use to create a fill-in box that will pop up each time I open a template, so I can change vital information which will be different for every contact.

Thanks in advance

The most elegant way to do this would be to employ a userform to populate document variables or bookmarks with the values entered into the form. The basics are covered at http://www.gmayor.com/Userform.htm.

The alternative is to use Content Controls in the document and enter the data directly into the controls.

Graham Mayor - MS MVP (Word)
www.gmayor.com

I agree with everything Graham said. I would add that you could use an Ask or Fill-In field. These are old technology, predating Word 97, but still work.
See the ASK field tutorial for examples.