In Ragnarok Online, you can enjoy two different PVP modes, 'Yoyo Mode' and, 'Nightmare Mode' which includes experience loss and item drop penalties. Pick the mode you which to try and join!
PVP mode is only available to characters higher than level 31.

You can enter the PVP waiting room with the help of the PVP doormen, located in the inns and pubs of each town, and a fee of 500 Zeny. Choose the room fit for your character's level and enter the room to begin the PVP battle. There is also a room with no level restrictions prepared for those "hardcore" players who wish to battle against players of any level.

The same rules apply in PVP battles as when in combat outside of the arena.

The rank is determined by the experience gained only in the PVP mode. It is displayed as 'Current Rank / Number of Players' in the bottom right corner.

PVP experience is calculated starting with a base of 5 points.
Each victory will give you +1, loss will apply a -5 penalty.
If you experience drops below 0, you will be expelled from the PVP arena. But if your experience is higher than 0, but you have fainted, you can ask a friend or player for help to regain your health and continue.