As Sorbek said, to enter PvP you just need to be on a PvP map.
PvP maps are anywhere that you come into contact with the opposite faction (other than AH(auction house) which is a neutral zone)
Let me try to list the pvp maps for you. i hope i remembered them all
15 Zone -- Proelium - entered through a portal in Keolloseu (AoL) or Gliter (UoF)
30 Zone -- Cantabalian - entered through a portal in Arktuis Vill (AoL) or Starfumos Vill (UoF)
1-70 Zone -- D Water Borderland. inclues...
Jungle (Stable Erde)
Desert (DD1- Deep Desert 1 and DD2 - Deep Desert 2)
1-70 Zone -- Valdemar Regnum (AoL) and Palaion Regnum (UoF)
60-70 Zone -- Kanos Illium
Linked Server (through portal in Apulune(AoL) and Iris(UoF))
60 Zone -- Goddess pvp map. has lvl cap of 60.
70 zone - Dwater and jungle maps
Obviously if you want to pvp its best to be closest to the max lvl of these maps as possible.
There is no area for new players to pvp. All of these areas require you to make very good gear in order to actually compete. You can go on the maps and meet the opposite faction, but expect to do no damage and to get killed in one hit.
this wiki has some information but it is outdated a little bit.