Welcome to Cantha

Hi
i just completed Welcome to Cantha..
but i dont get new primary quest..
so where can i get it?

Time to do Vizunah's Square mission

By completing the quest Welcome to Cantha, you should end up in the mission outpost of Vizunah Square. You then do this mission to progress through the story.

http://wiki.guildwars.com/wiki/Welcome_to_cantha

Not trying to sound like a complete ass, but are you paying attention? All of the posts I have seen you make so far seem to be asking for ways to move through the game fast. Play the game, and don't be in such a hurry, just enjoy it. If you don't enjoy it, find something new to do (PvP, another game, etc.). It is good you know about the Primary quests advancing you through the story, but don't be restricted to just those. Many of the secondary quests are quite fun, tell the story quite well, and offer nice rewards.

In all campaigns, the main storyline is a combination of Primary Quests and Missions. Generally speaking, if you don't have a primary quest, there is a mission you need to do.
If I recall, in Factions (and Prophecies) there is an "Enter Mission" button in the party box when you enter the outpost. In Nightfall and EotN, you speak to a particular NPC.

This.

This is a problem for many players working through the campaign backwards.