There are a couple of ways you can do what you are asking.
1. You can buy traffic (targeted or untargeted) and direct that traffic to content that the visitors might be interested in. Content must have a sign up page.
2. You can buy a list.
3. If you don't want to buy traffic, you have to have some killer content that's going to make people come to your site...you'll have to promote it somehow--advertising, articles, anything. then they'll sign up for your killer product
There are other ways, but these are the most common.