Cant play arena

[Mack897]

im pretty new to this game i got it on the day of release in the u.s but im having a problem playing the arena PvP and that's one of the main things i was looking forward to in the game, im a level 16 assassin on a NA server, and everytime i hit F9 nothing happens can anyone help?  (sorry if this is a dumb question new to the game like i said)