As a PhD student, reviewing paper is important for you to learn criteria of good papers and prepare your next career as a researcher. (You can put your reviewer experience on your CV.)
If it's your first time to review a paper, you would wonder how it should be. If you are asked to review a paper, you should have submitted paper(s) and gotten reviews. You would have noticed the review styles. Reviews can be written in this structure:
1) Summary of the paper
2) Contributions or possible contributions to the field. Weakness and limitations can be added.
3) How to improve the paper. E.g., structure and related work.
4) How to improve the minor errors that didn't affect your decision. E.g., misspells and figures.
More importantly, your attitude to the paper outweigh the structure. I'd like to recommend this paper to prepare the attitude: So You're a Program Committee Member Now