When you've found a good keyword, the next step is to create a new page or post on your website and create content focused around that keyword. If you think about the keyword, imagine what someone who is typing in that keyword is hoping to find. Then create content that matches this as closely as possible.
In most cases, this means writing an article, adding some images and maybe a video. Or, of course, you might want to hire someone to create that content for you.
For a lot more detail on all this, please take a look at the Rich Content Optimization guide:
http://www.swissmademarketing.com/media/downloads/SECockpit_SwissMadeMarketing-Rich-SEO_v1.pdf
For new niches/new websites, the process is not much different.
The tricky part is to find a starting point. I usually do this by browsing products that I can promote as an affiliate. I look at products listed in Commission Junction and Amazon, for example. I look for products that are more or less interesting to me and then I do keyword research, starting from those product names. When I've found a good set of keywords, I create a new site and one page for each keyword I found (again following the method described in the guide linked above).
