To modify your iPod Touch without Jailbreaking it, you need to turn it on and tap the Setting window. Then, scroll down the settings menu and tap the "Themes". Tap the "Themes Camera Roll" to get a folder containing a list of predownloaded images. From the folder, tap the themes and images you want to use to modify your iPOd Touch screen and select "Set" and you are done.
It depends on what you mean by modifying. If its changing the wallpaper, you should read sarahngoche's post. But if you want to change the look of the icons or make it look like android with dream board, you're going to need to jailbreak it.