Once you are in the jobs area click on the Get a job or work button to become a stablehand.
Note: A player can also only hold one job at a time.
You need to be level 5 or higher to apply for and have a job. The application fee is 3000 coins. If you resign the application fee is not returned. The Magic itemscalled the tome of labor will allow you to have a second job and work twice as fast on both.