The work you do earns you experience points. When you reach a certain number of experience points, you go up a level. With each level, you gain 1 Attribute point and 3 Skill points to allocate wherever you want. You allocate these by clicking on "Skills".
Jobs use 5 skills, which vary from job to job, and have a difficulty represented by a number. When the value of your skills for a job becomes higher than the value of the difficulty, then you can do it.
If you want to know how many more experience points you need to go up a level, on the top left of your screen there is a yellow bar with a star next to it. Mouseover to see the exact numbers. Or click on "Character" because it is listed there too.