One of the most important decisions you'll have to make as you decide to build the list is which niche you should focus on. There are literally thousands of niches out there, so there is no one right answer to this question.
You might have some idea of which niche you want to focus on if you have built up websites in that niche already. If you have no idea of what you should choose, don't worry! You can easily choose a niche if you follow the right guidelines.
The first thing you need to make sure of is that it is a hungry, or passionate market. This works well if people have some kind of problem they are desperate to stall. This is where people are passionate about their hobbies can work very well also.
It's a good idea to make sure people are already spending money in the niche you have selected. You can check to see if there are physical products, or digital products being sold regularly to this group of people. This is usually a good indicator that you can make some money as well. Another place to check is advertisements on TV or in magazines. Where there are ads, there is money being made!
Basically, you need to make sure you have something to offer a group of people. You also want to make sure the customer base will be large enough to make your efforts worth your while.
If you cannot think of a niche, you just need to brainstorm and work your way from there. You can go to Amazon.com, magazines.com, 43things.com, Yahoo Answers, and many other sites to get some good ideas. Write down anything that comes to mind or stands out to you as a potentially profitable niche to build a list in. Then, you can narrow your choices until you have some good options. Some further research should help you make your final decision.
Of course, as an Internet marketer you are not limited to choosing just one niche. After you have built up a profitable relationships with members of one niche, you can add another to your business. That way, you are not dependent on a certain group of people for your income. Spreading assets out this way is an excellent way to make sure your business always stays afloat.
Remember — never take the decision of which niche to choose lightly. Choosing the wrong one could mean you hardly make any money at all, and choosing the right niche could mean you make some life-changing income!