This application will allow you to set passwords to any application or folder in your iPod. Once you download it from Cydia (If you don't find it check our post on repositories here as you may have to add iphoneame) follow this steps to set passwords:
First you must acces to a new section that has been created in your settings called lockdown Pro. (as shwon in the image) the first time you access it it will ask you which password you want to set. It will also ask you for a safety question in case you forget the password.
Once in the settings you can change the password, safety word, applications locked or not and many more options.
You can also lock applications from the menu itself. Just hold an icon until all start moving, as when you move them, at it's lower right corner a grey lock appears. Click on it and it will ask you for the password you have set. Once you enter it the application will be locked and the lock will turn red. Repeat the process to unlock the application.