If you accept, you must treat the materials you receive as confidential documents. This means you cannot share them with anyone without prior authorisation from the editor. Since peer review is confidential, you also must not share information about the review with anyone without permission from the editors and authors.
First read the article and then take a break from it, giving you time to think. Consider the article from your own perspective. When you sit down to write the review, make sure you know what the journal is looking for and have a copy of any specific reviewing criteria you need to consider.