1st off PK'ing = Player killing. You go to a certain world (like 17 or 32) which are worlds that you can kill other players on. To PK, you bring food, armour and of course weapon/s. Its just like killing any NPC's. Click attack to attack them, and eat on low health. But beware, if you die, you don't have a gravestone so you can't get you're items back. Also there are a lot of clans out now-a-days, So don't go deep PK'ing.
Also, Because you're new to PK'ing, I suggest you 1st PK in Monk robes and a Rune scim (or anything like that) so you're not risking anything and make sure you go to a world marked with +1.
Hope I helped.