I feel like I've seen this somewhere on these forums before, but I couldn't find it when one of our members asked about it earlier today.
So here it is:
http://teamlegacy.net/_/gw2/sword-board-and-everything-in-between-a-deta-r622
Team Legacy wrote up a breakdown of what Guardian weapon skills are useful and when. Very good guide for someone just starting to consider playing a Guardian.