This info from the book might suffice up to mid-levels. Beyond that it apparently gets pretty hazy pretty quickly.
You set the task level when someone tries to Earn Income. The highest-level task available is usually the same as the level of the settlement where the character is located. If you don’t know the settlement’s level, it’s usually 0–1 for a village, 2–4 for a town, or 5–7 for a city. A PC might need to travel to a metropolis or capital to find tasks of levels 8-10, and to the largest cities in the world or another plane to routinely find tasks beyond that. Some locations might have higher-level tasks available based on the nature of the settlement. A major port might have higher-level tasks for Sailing Lore, a city with a vibrant arts scene might have higher-level tasks for Performance, and so on. If someone is trying to use a particularly obscure skill, they might have trouble finding tasks of an ideal level, or any at all—no one in most settlements is clamoring for the expertise of someone with Troll Lore.
Once the PC has decided on a particular level of task from those available, use the DC for that level from Table 10–5. You might adjust the DC to be more difficult if there’s inclement weather during an outdoor job, a rowdy audience for a performance, or the like.
Also, given table, how should i use the player check?
Let's say that on a task lvl 15, which I don't know how to consider in terms of location and so on, the player rolls a 33.
How do I know if this is enough or not?
This part I know how to answer! You compare to the DC for that level on Table 10-5 on page 503. The DC for 15th level is 34, so the PC fails (barely) and gets only the "failure income" from that other table.