There are a number of dedicated PvP Zones around Tibia. PvP is allowed in these zones, even on no-pvp servers.
Deaths in PvP Zones will not result in any loss of items, exp or skills.
All Guildhalls have a dedicated arena that can only be accessed by those who are invited into the Guildhall.
All towns have an arena where it is safe to train.
Thais Knights Guild
Carlin Knights Guild
Kazordoon Fighting Pit
Abdendriel Bug Pen
Edron Magic Academy
Darashia Knights Guild
Port Hope Knights Guild
There are also a number of PVP areas in the open world. These zones often require passing a protection zone to enter so they can not be exploited in PvP.