Mobile and other touch screens usually will get your fingerprints all over them, making it harder to see what you are looking at. The best way to clean these screens is with a microfiber cloth. For tougher to remove spots you can dip the cloth into a small amount of water and then gently wipe the screen. Don’t splash water onto it before cleaning, as water could get inside the device, which will likely void the warranty while potentially ruin internal components.
Some people suggest rubbing alcohol to remove fingerprints and disinfect the device. While this will be ok for some screens, many manufacturers recommend against it because the alcohol can eat away at the protective film on some devices.
If you notice that there is a lot of dust or gunk on the edges of your screen, or even in cracks, you may need to take the device into a mobile shop for further cleaning. Do not open the device yourself as this could void the warranty.