Your professor does not have a preferred citation style. If you are struggling to decide, a good rule of thumb is to choose a style that is consistent with the journals that you use most frequently. Make sure that you use the same style throughout your paper.
If you need help with citations, visit the USA Center for Academic Excellence. Consultations are free. The tutors can go through your paper with you to make sure your citations are correct.
There are several free citation generators. None of them are perfect, please always double-check that the formatting is correct. Here is a short list of some of those free generators.