There can be three aspects to this assignment;
1) Understanding the specific research that you had to read - by now you need to know that each study design has its own limitations and advantages and some study designs are stronger than other in relation to testing casual association but not always we can implement the best design. When researchers choose a design, however, they should try to make the utmost effort to increase the internal validity (i.e., use sound methodological process given the specific design) and consider the external validity - that is whether the results of this study can be generalised to the reference population. Obviously you need to familiarised yourself with the design of the study, what sort of biases are expected and how this study dealt with the biases if at all. Make a list to yourself on the weak and strong points of this study.
2) The other aspect is the reporting of the research. The study can be very good but very badly reported. You have slides that explain the structure of published papers and what we expect to see/read. This can be another aspect of your critique. Would you like to have more information that will help you assess the study but you were not given this..., or do you think the results presentation can be improved. Do you miss parts in the reporting of methods. Note that sometimes authors don't have a choice because there are limited words count for publication and each journal has its own rules, but authors need to set priorities and try to address the most important aspects, so you can check this as well.
3) The other aspect is the subject matter itself, the topic. You are not expected to show your expertise on this specific field. You are expected to find other relevant research so you can see what is known in the field, is the topic important to public health, is the issue still controversial or needs more supporting evidence... If there are specific issues that are relevant and related to the topic that you would like to raise you can do it if you provide reference to justify this. For example, if you think that the knowledge on this topic is already well covered you should back it up from the literature.
You are free to shape your assignment (i.e., how would you like to set up your headings and subheadings) it is all up to you. This is part of the marking - a clear logical flow is an advantage.