Ideally, information that shows up online won't contradict anything you've sent to the employer. Does your current or previous employer’s website still list you as an employee? If so, make sure the job title online matches the one you use on your resume. Do any YouTube videos you uploaded in college come up? You may not want employers looking at your student films and judging your taste level and skill from your early work. It may not be possible to alter your digital footprint too much, but if you know what potential employers see, you can think about ways to get ahead of it. And, if some really good things come up -- like the charity event you organized or your festival-worthy Vimeo short -- you can find ways to amplify those successes as well.
The key is not to overthink anything you might find online, but to be aware of what’s there. Look at yourself through the eyes of the hiring manager and understand that your online presence is a part of the fuller picture of your candidacy.