Write what you know about. Or at least on topics that you are interested in and would enjoy researching.

Website copy, depending on the type of site you have falls into two general categories. The copy on the actual webpages, and articles that round out the content of the site.
While all copy really serves the same two purposes, one to give information and two to have search engines find your site, the style of each of the types of copy on your webpages should be different.
The main pages of your site, where you talk about your product or your company, are intended to be “salesy” and promotional – think of these pages more like the pages in a promotional brochure or a sales letter.
The articles on the other hand are expected to be less promotional and more news or editorial in nature, but featuring the keywords that can drive traffic to your site.
Does this all mean you need to be a professional writer to handle the copy on your site. No not necessarily, does it mean that the most successful sites do hire professional writers for their sites? Yes many do. But if that is just not in your budget here are a few tips for writing acceptable Web Copy.