It has been said before that a Guardian tank is a bit of a challenge because of the trouble of holding aggro, among other things. I'm playing a Guardian tank myself and I'm following a build that has more skill points in Vigilance (a DPS tree) than in Defense (the tank tree). But I think that only shows the complexity and challenge that is a tank Guardian. If you go for a DPS Guardian (which is a safer bet and easier to get into for new players, I think), it might be best to focus on one tree and pick up a few skills from the other trees on the way.
But I think Jeff has a level 50 Guardian, so he might be better equipped to answer your question.