Before you begin, be forewarned that Word is a word processing program, not a graphics design / page layout app, so it may not be the best software for what you want to do :-) No insult intended, but the very fact that you had to ask this question suggests that you may not have a great deal of experience with Word, and this type of project can be one of the most demanding. If you have no other choice...
First, you can modify the documents generated from the templates. How to go about it is determined by which template you use & what changes you need to make - some are Publishing Layout documents, others may not be. Either can be altered, rearranged, reformatted in just about any way you wish, though. Content can be removed & other content added. They aren't much different than most other Word documents except that the layout & design work has been done for you - That's the tough part.
If you want to start from scratch I'd still recommend that you use a template. Business cards are best handled by a properly structured Table, but trying to do set it up yourself can be quite time consuming. Also, if the specs aren't perfect you won't be able to print the cards effectively (they're functionally the same as mailing labels). In Tools> Labels you have access to a variety of blank label templates from several manufacturers. Among them are layout templates for business cards.
If you search the ANSWERS forum using the term business card you should find some additional info.
Regards,
Bob J.