First click Bestiary button found in the client

Here you see all the Tasks being tracked automatically without interacting with NPC's.
Clicking the  tracks them specifically for you
You can search the creatures you have killed by clicking 

When you reach a milestone eg. */100 you will be rewarded automatically

A message will be shown in your serverlog.

The milestones are the following:
100,500,1000,2500,5000,7500,10000,25000,50000,100000

Its that simple! It all rewards and counts your kills automatically!