The internet is a public place, and whatever you post to it is out of your control. The one exception to this is encrypting your data, however. This will ensure that only you and the recipient will be able to read the data. Here are a few open source tools for public key encryption:
Sadly, though, the creator of GPG, Werner Koch, has been forced to give up maintaining GPG because he's running out of money and patience with the lack of privacy or people who even care. Leo encourages everyone to float him a few bucks through his online donation link.