Guardians are really good for catas because there an awesome healing char, which cata teams desperately need. But they also act as melee chars if needed since they can use heavy armors and clubs, so you can build it two ways.
If going healer; you can put the first 10 points to vit [or more] but since you are going to be healing constantly a higher mana pool will give you big life back, which is why you should go all intelligence. But I guess 5 or 10 points into vit wouln't hurt and only help. You should be looking for heal mastery or intelligence Club and heavy armor, it goes by preference on the stat because people will say that Heal mastery > Intelligence or vice versa. And of course your heal charm should have enhanced effect on it, as much as you can afford. That's basically it - you can then hop into a cata game and get praised.
If going melee; I would advise the first 10 points into vit, and then pump into dex or strength depending on your gear. You should have around 120 dex with your gear on at mq, and then all points should go to strength. Have either a dex plate and a strength charm or str charm/dex plate, but nonetheless the stats should be 120dex/rest strength. And lifesteal is a bonus add here, it's not entirely needed due to the update but it doesn't hurt. And your club should obviously have as much enhanced effect as affordable. And thats it,
The guardian isn't a bad melee char, but it works really well as a hybrid doing melee for the first 20 or so levels, and healing the rest. Overall an awesome char, good luck on your climbs