Pull the head,more than likely the head gasket is shot.Don't just replace the gasket and call it good though...really take the time to look over the head and cylinder for anything unusual.You will also need to lap the head and cylinder to get them FLAT or a new gasket will just blow again.
Don't cheese out on the gasket either!!...go with Cometic,or OEM...or again,you'll be doing this all over again much sooner than later.