Regardless of whether you use your own website template or a CMS like Wordpress, you need to have a great looking site to keep visitors from clicking the back button as soon as they arrive. You need to keep them engaged on your site to drive them toward a conversion that makes you money.
The primary factors in successful web design are usability and conversion optimization.
If you're selling a product you want your visitors to quickly and easily find exactly what they're looking for, along with marketing/conversion elements that make them more likely to buy. The same goes for affiliate and ad sites. You want your visitors to move through your site directly to the conversion, and it should be as easy as falling down.
One of the best books on web design is Don't Make Me Think. Every webmaster needs to own a copy. Text content can be very important on certain sites (like this one), and thinking isn't always a bad thing. But the central idea is that a visitor shouldn't have to think about where he or she needs to go to find something. It should always be front and center, and easy to find. The harder it is for a visitor to find what they're looking for, the more likely it is they'll leave your site without making a purchase or clicking an affiliate link or an ad.
Another very important factor in web design is search engine friendliness. Search engines can't read images, video, or flash. So you want to be sure all of the text and pages on your site are accessible and well organized, with category and page names that describe both to visitors and search engine robots what the page is about. When you think about site design you should also consider the code behind your pages. Well written title and description tags for example aren't only good for search engines, but also for the users who will be seeing them in the search results. For much more on this topic, see our pages on website promotion and SEO.
The design of your website should be largely based on the content and theme of the site. If you're using a Wordpress theme, certain themes are particularly made for showcasing photos, others for daily news, and others for static information pages. If you're designing a theme from scratch you may benefit by looking at popular Wordpress themes and the top sites in your niche for ideas. Before you even begin making your site, plan out the structural elements and navigation based on the content you plan to include, from categories and sub-categories to bottom level pages and products.
Many websites will require images of one kind or another. If you don't have images yourself, one of the best places to find them is iStockPhoto. iStock has everything from real photos to illustrations and computer graphics. Another great place to get ideas, free graphics, and great themes is Smashing Magazine. They not only have free graphics, but also great tips on coding.
If you want your site to be as valuable as possible, you should consider adding interactive content. Interactive content should be engaging and useful for your visitors. Great examples are quizzes, calculators, video, and free downloads. By creating engaging and useful content, your visitors are more likely to stay on your site longer, and share it with their friends.