This is highly inappropriate to ask you to do and lazy on the part of your mentor. I can say this because my mentor had me write a review as a 2nd year grad student and it was a horrible experience. Ask your mentor to defer this responsibility to a more senior grad student or do it himself. This is akin to asking someone to perform a difficult Bach piece after one year of piano lessons.
First of all, reviews are very lengthy (obviously), which requires not only significant knowledge of the field you are reviewing but how to write scientific papers. If you've never written one you are already at a disadvantage in this area and a review is a terrible crash course in how to write a scientific manuscript. Second, as a 2nd year student you cannot possibly have absorbed the information necessary to write a thorough review, and you will probably get some bad reviews based on that (some may be scathing). This is not your fault, but you should be prepared for that. Lastly, if you get it accepted, you will probably look back someday and feel a little embarrassed by the content of it. Again, it wouldn't your fault, but it still sucks to feel like you published something subpar and now have to live with it.
So in case the message isn't clear, my advice to you is not to do this. I gather from some of your posts that your PI is absentee, which is probably why he's deferring this off to you. But it's HIS responsibility and a bad review will still reflect poorly on HIM. He should man up and do his damn job. If you encounter resistance and still end up having to do this let me know, I'll help you however I can.