Removing cartridge from tap problem
Like Mike said, you need a special wrench to get that "nut" off your faucet. You will need something like this: 



You can get them pretty cheap at home stores or plumbing supply stores.  Trace out the nut on paper and bring it with you to the store so you can get a match.

It looks like someone already tried to remove it by tapping a screwdriver placed in one of the slots. This can work sometimes if the nut is not too tight but you always have to be careful not to damage the threads because some sleeves screw down on them. Good luck.