iPhones from 3GS and later offer hardware encryption; it is activated through the data-protection feature by enabling a passcode:
1. Tap Settings > General > Passcode.
2. Follow the prompts to create a passcode.
3. After the passcode is set, scroll down to the bottom of the screen and verify that “Data protection is enabled” is visible.
You should also encrypt your backup for added security. Check the “encrypt local backup” in iTunes if you back up to your computer. If you back up to iCloud it is automatically encrypted, but be sure you have a really good iCloud passcode.