Everyone's tax situation is different, so it is best to contact a qualified tax preparer or accountant for advice. In general, your stipend is considered taxable income because it is pay you received for services you performed. Marquette University will send end-of-the-year earnings statements to graduate assistants in January. They will be mailed to the same address that is on your check stub. If that address is not correct, notify the payroll office in writing so you will receive your earnings statements in time to prepare your income tax returns. Tuition scholarships generally are not considered taxable income.
Generally speaking, payments for services are taxable. There are some special exceptions for FICA as opposed to income taxes, but the Payroll Office handles those as applicable.
IRS Publication 970, Tax Benefits for Education, provides further information. It can be downloaded at www.irs.gov.