WordPress (Wikipedia, 5/31/11): "WordPress is an open source blog tool and publishing platform powered by PHP and MySQL. It's often customized into a Content Management System. It has many features including a plug-in architecture and a template system. WordPress is used by over 14% of the 1,000,000 biggest websites."
First Thing's First
Pick a topic. Make a firm choice in your area of expertise because you only get one. Let me say it another way: if you try and build content as an expert for ten different sites, with ten different topics, it will not work. Your time will be wasted and you will walk away defeated. I cannot emphasize this enough.
Decide on a domain name. You are better off with a short, brandable domain rather than a long-hyphenated-keyword-match.com domain. If you can get something short and unique with a .com that's perfect, but you can still get creative otherwise. For example you have probably seen people use actual domain extensions as part of the branding (think del.icio.us or shopsocial.ly).
Find a web host. There are plenty to choose from, and if you're just starting out all you'll need is a shared hosting platform...I use Hostican.com. Lucky for you the shared hosting providers are more similar than they are different, but you should still spend a bit of time looking around the web for consumer reviews of the different hosts.
WordPress & Configuration
The inherent nature of the WordPress CMS allows for easy categorization, tagging and archiving of your web content. Content can include anything from photographs, editorials and articles, to podcasts and videos. As Gary V says, "just make sure and put out good shit." Be authentic. Be yourself. Be the authority on your topic. Build your personal brand online. Whether you're a true authority or just a poser will come out clear as day in the content you put out.
Most shared web hosting platforms have one click installs of popular tools and scripts via admin interfaces like cpanel. If that's the case with your host (this might be something to find out before you choose on a host), installing WordPress literally takes seconds. If that's not the case don't sweat it because installing WordPress manually is not that difficult. Start here.
Once you have WordPress installed find a theme (or skin) that works for you. There are countless themes out there, but use this Free WordPress Themes database to search from. You can also find higher quality paid or custom themes, but to start out get a simple layout that's free. If you have some basic HTML or CSS skills than it's fairly easy to do some of your own customizations.
Now that you have a theme, tweak two important admin settings before you get started from within the WP panel: Permalinks & Privacy. More info on each below, including screen grabs.
Settings -> Permalinks
You can assign a custom permalink structure to the content you produce, and you should. Personally I think it's best to structure URLs to include the keywords you use in page or post titles, as well as categories. To do this simply go to "Settings" then "Permalinks" page, select "Custom Structure" then apply the following value to that field:
This allows permalinks (or URLs) of the content you produce to include category and postname keywords. So for example if I posted a fact on GeekArm.com about how bad it is to try and build ten thin Affiliates sites at once, the title might be something like: Focus or Lose. With the custom permalink structure in place, the archived URL would include the category (Facts) and the post title (Focus or Lose): http://www.geekarm.com/facts/focus-or-lose/
WordPress Permalink Settings
Settings -> Privacy
If you're building a niche Affiliate site it's essential that you allow search engines to get in and index your content. There is a specific setting for allowing or disallowing this on the "Settings" then "Privacy" page. Real simple...just select "I would like my site to be visible to everyone, including search engines (like Google, Bing, Technorati) and archivers" and then click "Save Changes".
WordPress Privacy Settings
If you own high profile SERPs with good content, for competitive KWs/product searches in your niche, you will convert sales and earn commissions. Of course the trick is going to be to win those SERPs, so dig in and do it right. You will not succeed by gaming the search engines. Build something from your knowledge and passion that's sustainable.
There are a handful of plugins that you should download and activate right away (see arrows below in "WordPress Plugins List"). As you get more familiar with WordPress, and become more familiar with it's plugin capabilities, you will be able to easily add or delete others. There are lots to choose from and you can read reviews or view ratings before you download and activate them. In the dashboard sidebar of WordPress just click "Plugins" for access to your list of plugins.
WordPress Plugins List
Akismet should come with the install, and yes you should activate it right away. It effectively controls and filters comment spam that will become rampant once your site gains some traction. You'll first need to sign up for an API key, though, and there is a small monthly fee...no complaining, it's entirely worth it.
Others I recommend starting with include a video embed plugin of some sort (there are lots!), WP Super Cache and ShareThis (or a similar plugin that helps others share your content). Downloading and of course activating plugins can be done from within the WordPress admin panel by clicking "Plugins" then "Add New".
WordPress Install Plugins
Remember it's not brain science to follow the guidelines above for WordPress. Your two main challenges will be to 1) focus your efforts, and 2) dig your heals in for the long haul. Do yourself a favor and focus on your area of knowledge or expertise with one web site and put out good content regularly. If you do this you will earn natural inbound links over time, which naturally builds site authority...preserving the integrity of your efforts to build a profitable, niche Affiliate site.