You are considered a self-employed person and you must file a return if your income from self-employment is $400 or more. Assuming you are single, you will not be subject to income tax unless your gross income is over $7,200 (and even then, if you have certain deductions, you would owe no income tax).
But you will have to pay self-employment tax (Social Security & Medicare). If you earn over $400 from self-employment, your self-employment earnings are taxed at an effective rate of a little over 14%, see Form 1040, Schedule SE, which you can download at:
http://www.irs.ustreas.gov/forms_pubs/index.html.
Also, I would suggest you download, from the same site, IRS Publication 17, Your Federal Income Tax. That publication will tell you everything you need to know about the income tax and the self-employment tax.
