If you're not looking to build a multinational online media conglomerate, then don't worry too much about targeting specific demographics. Instead, focus on building your site around things that you feel close to or care about—but without making it too personal. For example, you're not likely to get enough traffic to your site to generate any revenue from advertising if you're simply posting pictures of your kids. If, however, you were to base your site on providing tips to parents for bringing up kids to be happy and healthy, you'll stand a better shot of building an audience and attracting brands that want to market to that audience (childcare centers, baby-food or toy manufacturers, etc.). If you love sci-fi movies or model trains, consider building a site that provides information on those topics—upcoming events, recent news, tips, or opinions. The nice thing about keeping it small is that your site doesn't have to be too business-like. You can enjoy it, write about what matters most to you, and build your audience with people who share your interests. You, your viewers, and potential marketers will all benefit from it.