Leigh Honeywell wrote an excellent blog post for the ACLU about staying safe online. It's mostly a primer for people who need a quick jumpstart into how to keep themselves safe, but be warned, some of it takes some time, and won't be 100% effective. For example, scrubbing your online presence is a lot easier if you've never maintained much of one - search engines cache and don't always clear. If you've forgotten about accounts with re-used passwords, those can still come back to bite you.