If you have work-study as part of your financial aid package, you will get access to the work-study job listings. From there, you basically just submit your name, uni and check off that you've turned in the relevant tax forms and the employer gets back to you for an interview. Depending on the position, you may be asked to submit a resume; these are typically for more specialized jobs e.g. translating documents for a professor's academic paper or working on a research project in a lab. Beyond that, there's an abundance of "office-*****" type jobs where you fax, copy, file and run errands, jobs swiping IDs at security desks and shelving books at the library.
If you want my advice, avoid the office work jobs. They usually keep you busy running errands and doing gruntwork which makes it difficult to just sit and read or do homework. For the pay, you might as well just swipe IDs or work as an usher at Miller Theater. Also, when on the Federal Workstudy site, the postings that are most recent are the best to apply for; usually people fill positions and are too lazy to take the listing off the website so some of your applications to older postings will just go ignored.