Your first source is obviously the search engines, such as Google, Yahoo! or MSN.
You should also consider accessing business directories online, such as Hoovers, Dun & Bradstreet, Leadership Library, Reference USA, Corptech, Datamonitor, Mergent, and Plunkett Research. These are good resources to understand a company’s ranking in its industry, its financial strength, its executives and their backgrounds, industry trends, competitors, and customer base.