/smartass nitpicker mode on
It's spelled "C'tis." Pronounciation is a whole other subject.
/smartass nitpicker mode off
Seriously though, I'd always imagined the C at the start to be pronounced as a K. What with Celts and so on. Dunno why the connection's like that in my head but there it is. So it'd go something like "k'tees" by my reckoning.
Then again, I'm not a native anglophone either, so what do I know?