This doesn't sound very secure I know, and it's not meant to be super secure. It is more meant to provide a way of proving it is a legitimate email from you, since only you hold your private key. To get a secure message to someone you use their public key to encrypt the message, and then sign it with your private key. When you encrypt something with a public key, it can only be decrypted by the private key. The reason you sign it with your private key again is the same as before, this verifies that it is really you sending it.
Here is a diagram to help make these steps make since.
What's lucky is there are some great tools out there that makes adding this security easy and hassle free.
For Windows users I suggest gpg4win. They have lots of documentation and installation guides. The package even comes with an open source email client with built-in gpg features. I suggest you use a more familiar email client, Thunderbird has a gpg add-on that makes it super easy to use gpg too, plus thunderbird has an auto-configure tool that sets up your email for you.
For Linux users there is gnupg which can be installed from the terminal on Ubuntu with this command:
sudo apt-get install gnupg
After you install this go to gnupg's howto guide and follow the directions to set-up your first gpg key pair. Then if you haven't already install Thunderbird and get the gpg add-on just the same as with windows.
So, go out there and get secure and protect yourself from prying eyes!