How do I get my money back if some of my paycheck was withheld for taxes even if I'm exempt?

I'm a student working a part-time job and I filled out the papers I needed to fill out in order to get paid, which included some tax papers where I claimed "Exempt." The problem is that I didn't write a "0" in a specific box and they withheld $20 from my paycheck. I do qualify for exemption but I just filled out the tax papers wrong. I went back and fixed the paperwork but, I was wondering, how do I get this money back? The woman at the Humanities Department told me to file for a tax refund in the spring semester but didn't go into details. How does this work? I know its not a lot of money, but for a college student, it actually is. Can anyone tell me what to do now?