From the temple of the main continent, head south and enter the Creatures/Quests teleport, as indicated in the image on the right. Once inside, go up the two staircases, and you will immediately find the first NPC: Snip Task Master. Talking to him will open a small menu. Simply choose the Task option to begin.
When choosing the tasks you want to do, there will be three options for you to select: Easy, Medium, and Hard. Each difficulty represents the number of kills you will need to complete for that task, with harder difficulties giving more experience and items in return compared to easier ones.
As shown in the image on the right, the difficulties are on the left, under the name and image of the monster, and the rewards for each difficulty are displayed on the right.
After accepting a task in the difficulty of your choice, you can now head to the spawn and start killing monsters. Don't be afraid to create a party, as you can share task progression with three other people, as long as everyone deals at least 1% of the creatures' health as damage. Once finished, simply return to the same NPC where you received the task and turn it in. Also, elite monsters provide extra kills for progression.
Be aware that after completing a task, you will not be able to do it again for a certain period of time. This cooldown is different for each difficulty selected, meaning a higher difficulty will have a longer cooldown. In general, it is recommended to do easy difficulty tasks if you have free time to stay active and medium or hard difficulty tasks otherwise.
Completing certain tasks will grant you access to the specific task boss, if they have one. By talking to the task NPC, select the boss room, and you will be able to see which tasks have a boss. The first available task boss is the Giant Spider.
Killing one of these bosses grants you present boxes, which contain various items such as gold, upgrades, and a chance at some relics if you're lucky.
After reaching level 150, you can proceed to the new zone with stronger monsters, along with a new task NPC. Head north from the Snip Task Master and enter the door, which takes you to the new zone. Danny Drinkwater will be the task NPC for this area. He also has a shop that sells Task Boss Charges, which allow you to face and potentially defeat task bosses without needing to complete their task. The currency used for these is task points, which you earn by completing normal tasks.