Changing apple id on iPhone

Hi, my husband & I both have iphone 4's which have been great up until recently when i was given an ipad for Xmas.

I have set up icloud on my Mac, iphone & ipad but find that when i send or receive a message my husband also gets a copy of this on his phone which is a pain. His phone was set up under my apple id when we first purchased them. If i register him for his own apple id how do i change this on his phone???? & will i still be able to sync both phones on my Mac so that i only have to purchase songs, apps etc once.