The Sony Xperia S uses the new smaller micro SIM cards.
To install/insert the SIM card and get full use of the smartphone, you need to insert the SIM like so:
1. Remove the back cover using your thumbs, press down on the back cover and slide it upwards.
2. Insert the SIM card into its slot (see bottom image) with the gold-coloured contacts facing downwards.
3.To replace the back cover, place the back cover over the back of the phone and fit it into the slots. Slide the cover downwards (the opposite to removing it) until you hear a sound and feel the cover lock into place.