Esteftah - reviewing submitted manuscripts is a skill that takes time and practice. I review for around 40 international journals - including sitting on several journal Boards as Associate Editor etc.
For the last few years I have been a member of the company Publons - which promotes open review. This is a good resource, if you are not signed up (and it's free) for learning and observing effective review process. For the last year, I have been an invited member of their Publons Academy - as this link https://publons.com/community/academy/
This involves me 'mentoring' new reviewers and 'reviewing their practice reviews' before they send back to a journal. It is another good way for new reviewers to learn.
My other piece of main advice is, look to the feedback that you get when you send manuscripts in for journal review. Ask yourself - what is useful feedback for you - and what is not? For instance, for me, the best feedback is honest, balanced, constructive and comprehensive across the whole manuscript. The opposite doesn't work for me.
When writing the review, be open minded with no bias, be objective about the clarity of the article, the novelty of the work and contribution(s) to knowledge. Having read through the methodology and it is captivating enough and the results and discussion section with all other sections are also thoroughly read, then you must start with a statement to clearly show the novelty of the work to display a constructive criticism of the work and then followed by your reservations on every aspect of the paper and also helping the authors through your comments