Do not be afraid and do not be embarrassed. You are not a literary critic, and this is not a project of philologists and literary scholars. You have the right not only to your point of view, but also to its presentation, but for a competent presentation. 

What not to do

1. If you are not a master of improvisations, do not write a review “on the first call.” By this, you will save yourself and others from numerous problems and greatly improve the quality of your reviews. Think about it.
2. In the continuation of the previous one – do not write reviews directly in the form on the website. It is better to use a Word (or simply paper). This will allow you to express your thoughts without panic and fuss, to edit (and not just) the written text, to correct mistakes (this is mandatory!).
3. Do not retell the plot of the work. Those who read the work, they already know it, and those who did not read, will not be happy with your retelling. 
4. Do not repeat what is already written in other reviews. Meaning? It is better to unsubscribe in the comments. Similarly, do not write a review, the only purpose of which is a polemic with the author of the previous review, for this, there are also comments.

What to do is worth it

1. You must clearly understand – what exactly do you want to convey to the readers. It is desirable to know this before writing a review, and without fail – before its publication. 
2. A good title is half the review. Do not spare the time to think about it. 
3. the most important parts of your review are the beginning and the end. Firstly, because only the beginning is displayed in the list of reviews. Secondly, because these parts are most memorable to readers. Work them out carefully.
4. Argument! You can argue anything: that “Eugene Onegin” is a mediocre work, which Shakespeare is a postmodernist that “War and Peace” is about aliens, just argue your point of view.