Prior to asking someone for a referral it is important to find out who exactly would be the right person given the context of the situation and/or position. Most employers like to conduct 360 references meaning a manager, a coworker, and someone who reported into you. However, in an entry level position it would also be safe to have a coworker/classmate and a past manager/teacher. Once you have identified who could speak on your behalf in an honest as well as positive light, it is important that they are comfortable being a reference for you and that they are aware of the position you are applying for. The next thing would be giving them a time frame of when to be expecting this conversation. If someone is not completely comfortable taking a phone call, then an email or written document can be used as well. Best of luck!