Here is a look at how to lock the screen on an iPod Touch with the Passcode.
iPod Touch Security
Putting safety measures on your iPod Touch can prevent it from getting stolen, or just limit those who are going to play with it; for example, if you have small children that you want to limit access, do not want people to make iTunes and App Store purchases on your iPod Touch, or simply do not want party guests to change the song when it is plugged into a speaker system. To do this you essentially lock the screen on an iPod Touch so that people can't get past the opening screen, and therefore cannot interact with its features or included applications. This is a built in security feature on the iPod Touch and can be applied easily.
Once you have set the Passcode Lock on your iPod Touch you will have a few settings you can change. First, you can go to the Require Passcode and choose how often you will need to set the iPod Touch's Passcode after the iPod Touch goes to sleep. It is set for Immediately, but this may be a little too quickly for some users as it will require you to enter the iPod Touch's Passcode often. If you want to set an even more complicated iPod Touch Passcode turn off the Simple Passcode. You will then have to enter in your old iPod Touch Passcode, and then you will be able to set a much more complicated iPod Touch Passcode that can be more than for digits and can include a whole variety of characters.
Locked iPod Touch Screen
Now when you try to enter into your iPod Touch you will be forced to enter a Passcode as there is a lock on the iPod Touch's screen. Once you enter in the iPod Touch Passcode correctly you will be able to remove the lock on the iPod Touch's screen, but only until the next time you have to enter it. The iPod Touch Passcode is a nice iPod Touch security feature, but it can make going back and forth very difficult. This actually becomes a major problem if you are listening to music for a long period of time as you will have to enter in your Passcode just to stop the playing of it. Since your iPod Touch is not connected to the internet at all times, like the iPhone, you do not always need a security feature to stop people from downloading expensive apps or media.