Enter the Fight Club in North Qeynos by entering a cellar door ( 406, -25, -50 ) /waypoint 406, -25, -50 at the Bell platform in the Qeynos Castle moat.
To get to the Fight Club from Antonica, -> Peat Bog -> Vermin Snye -> Down Below -> North Qeynos.
If you're high enough level, you can enter NQ directly -- most guards are 55, although there are some wandering epic 95 guards on horseback.
If you came via the sewers, you will come up just south of the drawbridge to Qeynos castle. Jump on the sewer grate, then onto the wall (you should be able to catch the slight ledge on the city wall), and then over the smaller wall to reach the moat. Swim north in the moat.
Look for the bells just north of the drawbridge, on the landing at the base of the stairs coming down. There is one guard and a dog on the north end of the barge here, just stick close the south side and you will see the Fight Club entrance.
Once inside you will have 4 guards, 2 on each side of the room(can be up to 6). Dispatch or sneak past them and head into the side room with another guard and an awaiting contestant. Kill the contestant for his arena ticket to get into the arena.