Coop can be a bit difficult if you want a specific person to play with. You have to buy a ring from a character in the starting area and make sure you are both close to each other in soul memory. It's easy with randoms, though. People can lay down a summon sign and if you haven't beaten the boss of the area, you will see them when you are human. A smaller sign will only summon someone for about 5 minute and then they will disappear. A larger sign will be summoned until they die, disconnect, you die, or the boss is defeated. White and gold signs are coop. Blue and grey signs are PvP.