A proper spark plug socket is an absolute necessity.
When removing old plugs, you want to keep "downward" (i.e., toward the plug) pressure on the tool to keep it from slipping off. It sounds to me like your socket slipped off the plug while you were pulling on the ratchet/breaker bar, and broke the porcelain.
A couple of good taps on the breaker bar has always worked for me to break the plug loose -- but don't overdo it. These are aluminum heads, after all. Once you break the plug loose, do as others have suggested if you encounter resistance: 1/2 turn to the left, 1/4 turn to the right, 1/2 turn to the left, and so on.
Before you install your new spark plugs, put a small dab of anti-seize compound (I've always preferred the copper stuff) on the plug threads. This will prevent the plug from seizing in the head. After a couple of heat cycles (100-200 miles), I always re-check the torque on my plugs.
2013 Billet Silver JKU Sport S, 6-Speed, TF Leveling Kit, Spidertrax 1.5" Spacers, 285/70R17 Firestone Destination M/Ts on OEM Sport wheels.