how do i get capture signet.

someone told me that u get from quest but what quest. if not how?

http://gw.gamewikis.org/wiki/Signet_of_Capture

Read "skill quests" and pick the one appropriate to the origin of your character.

^ what he said.

You can always buy it from the skill trainers in various major cities/towns and outposts: i.e. Lion's Arch, Kaineng City, Kamadan.