All PvP (player versus player) combat in Ryzom is consensual - that is, you cannot be engaged in combat with another player without agreeing to it in some fashion (excepted in Primes Roots and Nexus, PvP areas). There are several ways that this can occur however, so if you are looking to engage in PvP you should be able to find other willing participants.

Duel

By challenging another player directly to a duel, you can engage in individual combat even in areas where PvP is normally not allowed. To challenge another player, right-click on them and choose Engage Duel from the context menu - they will receive a challenge, and if accepted, the duel will begin. Once engaged in a duel, other players will not be able to heal or assist you in any way - duels are limited to the actions of the two players involved.

The duel will end when one player is slain, or if either of the two players abandons the duel. To abandon a duel, right-click the player you are in the duel with and choose Abandon.

Arena

You can find one arena in the Matis land, completely open PvP within its borders. As you near this area, you will receive a warning that the area has open PvP enabled, and an opportunity to turn back without being vulnerable to attack.

PvP Challenge

For a more advanced form of dueling, two teams can use PvP Challenges. To begin a PvP challenge, the leaders of two teams must agree to engage in a PvP Challenge, using the same procedure as beginning a duel.