Please choose a published paper from your field of research to review. Pre-prints (such as those from osf.io) are excellent papers to practice reviewing as they have not yet gone through peer review but are considered published. Try and choose a research article instead of a review paper, as you'll get more out of the course that way.
You cannot review papers that you have authored as you'll be biased, nor can you review papers you've already reviewed for a journal as this would go against the journal's peer review confidentiality policies. In either of these cases you'll be asked to choose a different paper to review.