first you need to submit a Free Application for federal financial aid. www.fafsa.ed.gov. Work study is based on need. Make sure you answer yes to work study on the applicantion. The college will usually consider you for work study if you qualify.
If you did not select on your FAFSA that you would like Work Study, a Financial Aid Administrator at your College should be able to review your Need to see if you have room in your budget to be awarded a Work Study award.
Job listings should have a contacting manager. Give them a call let them know you are interested to setup an appointment. If they give you the ok they should have next steps or redirect you back to financial aid office or student employment office.
Ultimately the financial aid office will handle the award and update your award letter if approved. Paychecks and scheduling will most likely be handled from other departments. I always suggest starting in the Financial Aid office or their website for more information.
Jonathan recommends the following next steps: