Is it possible to delete your search engine trail?

Alright, so I'm a big internet user, and a member of many forums. I am forgetful, so I tend to use the same user name for everything. A friend of mine did a google search on my user name in jest, but the amount of information that came up scared me. Basically, any forum or journal post or membership of any website I have ever made came up in the search results; available to anyone who was looking for it. I'm only a teenager, so I don't like the idea of having this much information about me accessible to everyone. Is there a way to make my information more private, or would I have to manually delete everything from the websites they were originally on? Thanks in advance.