How to supervise an undergraduate student if I am a PhD student coming from a different field?

I have joined a new research group as a PhD student about one month before. Few days ago my supervisor told me I should supervise an undergraduate student for a research project. However, the things this research group doing are totally new to me. When I had discussions with the undergraduate, I sometimes find he knows more about me in the project. I find embarrassed, and don't know how to supervise him when I can't quite understand his project.

I know as a PhD student, learning new things is essential (and I enjoy a lots). However, I do not have the time now. The undergraduate needs to have some results in few weeks for some sorts of mid-term review report. I agree to supervise him because I don't want to reject my supervisor, I just join the group and it seems inappropriate to turn down.

What should I do in this situation? I don't know if it is common for a fresh PhD student to supervise undergraduates even you are coming from a different field. Of course, I am doing a lot of reading for this, trying to beef my background knowledge up.