You will first need to have a business that you can earn income through Internet (Online retailer, online service etc). Once that established, you will need visitors to your site. You can do that by using Adwords to get visitors to your site. Afterward depends on how successful you run your business to get good income. Here's the Adwords beginner guide: http://adwords.google.com/support/aw/bin/static.py?hl=en&page=guide.cs&guide=21899
One way would have a clickbank account and make a site about one of their products. Then you use PPC and advertise the product.