Start With Social Media
The place we tend to leave the deepest impression, over time, is on social media. Consider social media the "heel" of your digital footprint; it's where we tend to leave the most weight. Most social media behavior is fairly innocuous, but look over a few things. For example, you may have "liked" a page or business years ago that you no longer care about, or perhaps has a bit of an ugly reputation now. Or there might have been arguments you've gotten into, or things you wish you hadn't said, that are posted for all to see. Take a moment to find those things, clean them up, and apologize to those involved if necessary.
Check Your Website Comments
Another place you might want to clean up are comments on your favorite website. Take a moment to read over what you're saying, even if it's not under your own name, and ask yourself if it's something you would say in person. If not, consider deleting it; you never know when your comments may stop being anonymous, and those comments may be tied to your email address.
Still, they might contain data that you'd rather not have public. Or you might be uncomfortable with not knowing who's following your activity online. So consider deleting them, especially ones that you're not sure where they came from.
Examine Your Browser History
Many people aren't aware that most web browsers, as a matter of course, store the websites you visit in a history. That's so if you want to find them, you can easily return to them. However, that history can give away deeply personal information; if you've been researching a medical condition you've been suffering, for example, every site you've looked at will be in the history. Cleaning out your browser history is a good way to ensure your digital footprint doesn't impinge on your privacy.
Finally, the best way to keep your digital footprint clean is to not get it dirty. Think carefully before getting into an argument online, even with friends and family. Before posting a comment, ask yourself if this is something you want tied to your name. And above all, remember the old adage that a closed mouth attracts no feet. One of the best ways to keep your digital footprint tidy and a smart rule of reputation management in general is to simply think carefully before you speak.