Once you crumple a paper, it's hard to smooth it out. Same goes for your reputation and your relationships. Make sure what you're posting won't hurt a friend. It's hard to rebuild a friendship after you damage it online.
Take a look at what you've shared online. Do you like that person? Sometimes kids think they have to be cool online, and they post things they think other people will like. Be your authentic self--it's much easier than maintaining an online presence just to fit in.
Everyone makes mistakes. We've all done something we wouldn't want shared online. But your world IS shared online, and can be easily Googled. Try to keep your online mistakes small enough for a band-aid, and not surgery:)