You learned all you needed to know about lak from the quests on Trainee Island didn't you?
You must be wearing a Chaos Necklace to collect lak after defeating mobs. Not all defeated mobs will drop lak. Your lak carrying capacity is displayed in the Alt+I Inventory window and is determined by the level of your necklace.
1000 lak can be traded for 250000 rupee
or for the following boost chips (cost of 1 chip when changed for lak in brackets):
Type Rank I Rank II Rank III Rank IV Rank V Rank VI Rank VII Luna 2403
Spirit type monsters, such as Salamanders, Pixies or Angels drop considerably more lak than others, however, also more rarely.
Lak may be exchanged at the Soulcrafter NPC in exchange for a recharge of Soul Power on your or your pet's equipment.