You can use a lint free cloth (there is one included with the iMac packaging). You can damp the cloth with water if required. It's difficult to scratch the screen, so don't worry about putting a little effort in. See also:
Use a damp, soft, lint-free cloth. It is also safe to use isopropyl alcohol 70% on the enclosure parts. Remove any surface dirt gently with your bare hand before proceeding with a cleaner and cloth. After cleaning, dry the plastic or aluminum with a soft, lint-free cloth.
iMac
Important: Do not use isopropyl alcohol or a similar product on the bare LCD panel. Use only a damp, soft, lint-free cloth or LCD cleaning product.