Choosing a good niche subject to base your website around is one of the most important aspects of making money off of your articles. This will give you a foundation to build from and you can target one general audience with a pack of keywords that they are most likely to be searching for. You should take each one of these keywords and use it for the basis of one article on each page. This way, even though you are targeting one specific subject, you will be sure to interest a wide variety of people in that one niche. They may also find other pages that interest them, which will keep them returning to your site to learn new information about the niche subject.
The best way to find keywords for your niche subject is to use a keyword software program and type in the word that is the subject of your niche. This will generate a list of keywords or phrases that contain your niche and will also show you approximately how many people search for each word or phrase. Some software programs may also tell you how many sites are out there to compete with for each word or phrase (this will help you know if those sites are worth competing with for the number of searches out there). You then decide which of these would be most profitable by determining which have the least amount of competitors, so that you have a chance at making it to the first or second page of search engine results, but that also have a decent amount of people looking for that keyword or phrase each month. These will be the keywords or phrases that you will base the pages of your niche site on.
If there are a number of topics that you like, pick the one that you feel would be easiest to start with and then, once that site is built and generating some revenue, you can start another site. The most profitable website marketers, who use their talent of finding niches and combining that with good site content and a handful of affiliate links, have a good amount and variety of niche sites that they have started. You are never limited in what you can do with niche website marketing, unless you find out that you do not have the marketing skills or the needed funding to make it happen. Otherwise, the sky is the limit!