Summarize:

Select the “General” tab from within the Properties dialog, followed by “Advanced”. In the “Advanced dialog select “Encrypt contents to secure data” followed by “OK”. Open the Finder and navigate to Applications>Utilities> and double-click Disk Utility. From within Disk Utility, select File>Blank Disk Image. Enter a name in the “Save As:” field and select “128-bit AES encryption (recommended)” from the “Encryption” drop down menu. You can also browse to select a specific location to save the new disk image. Click the “Create” button. Enter a password and verify a password for the disk image and click “OK”. A new “.dmg” disk image will be created. Double-click it to open it as a virtual drive. You can now simply drag and drop files and folders onto the virtual, password-protected drive to keep them protected when on a Wi-Fi network.
Encrypting important private files and folders is a great method of keeping them from prying eyes that may have been able to access your machine through a Wi-Fi network. On Windows, right-click the file or folder you want to encrypt, select “Properties” from the context menu. On Mac OS X, you can encrypt files by creating a virtual password-protected drive to store sensitive files and folders.